I think this is mostly a problem for people who attend Mass with the SSPX or some other group (or independent) but also have an interest in attending a Bp Williamson conference. While this is not necessarily unreasonable, you should come to terms with how these things play out and make the effort to go to a Resistance Mass every now and then so that you'll know when the bishop is in your area.

I doubt you're going to have much luck in changing the way the bishops operate by complaining about it here.

I'm quite thankful that Bp Williamson posts most of his public appearances on YouTube so that I don't have to travel across the country to hear his sermons.

You make the point.  If you are not local or privy to a 'bulletin' you are not invited.  Isn't this a resistance website making all kinds of announcements from everything under the sun to some really outlandish stuff?  And there cannot be a simple announcement to help these priests and bishops?  Even us benefactors are in the dark where they go.  Secret emails is for the timid.  Gives doubt what the intentions are from those people who know and want to hide the joy of their visit.  Yet, this thread asks if anyone is going to the party of Bishop Williamson tomorrow, which I would like to, but how could we, there is no foreknowledge but a day.  Still there is no address for the event center even when requested.  These bishops and priests are more than a quaint passing.  Aren't they?  Why are we having these same discussions EVERY TIME finding out either days before, or most often, after the fact.  How are we to be united if this continues with realistically secret masses and events?  This pfeiffer excuse is for the birds.  Where is the faith?  Where is the charity to help your neighbor?  I don't want to think this is a cult.  But this is sure weird.

The building of brick/mortar chapels would solve this problem: Nobody would be able to get a venue cancelled in such places.  This would also remove reluctance among the various chapel coordinators to have their homes (i.e., chapels) broadcast over the internet as targets for troublemakers.  And obviously, as an edifice (i.e., consecrated space) is essential to render proper glory to God, newbies or the less principled would not be deterred from attending Resistance venues because they are "weirded out" by attending Masses located in private homes.  Therefore, I suggest this goal could be a logical first step.  But which locales have sufficient numbers/finances for this?  And even if they did/could build something, there are no priests to staff such chapels.  If we build it, will they come?  Not so sure.  Perhaps Bishop Fellay will help us out in that regard next week.  Meanwhile, perhaps a "via media" would be for the the bishops, priests, and chapel coordinators to build a master email notification list.  This would have the benefit of broadening notifications to people outside our own immediate chapels, without drawing the unwanted internet scrutiny?  It does not remove all danger of indiscretion or treason of course, but since when was 100% security a prerequisite for the apostolate?
In any case, the response quoted above might provide a reasonable response for the secrecy surrounding Bishop Williamson's travellings, but what about the other three bishops?  Surely they are not as despised and targeted as Bishop Williamson, but unless I happened to have Bishop Zendejas' personal email address or phone number, or happened to know someone at the Resistance chapel in St. Mary's, how would I ever know where or when Mass was?
An email recently came from Santa Cruz which contained the names/email addresses of approximately 250 persons, to notify them about a building project in Brazil.  I propose the American Resistance compile a similar list of known collaborators in the USA for notification purposes.  Santa Cruz demonstrates such a task is simple, and even though there were many names of known critics and enemies of the bishops on that list, still, to date there have been no deleterious consequences despite the fact.  I think this would be a reasonable interim solution, until such a time as we can build brick/mortar chapels, and be done with these covert ops.
This is just my idea; there may be problems and issues which I have not considered; I am just brainstorming for ways to improve this bad situation.  I suppose if you live near a chapel which has frequent/regular Masses, you think this is no problem at all (i.e., You are always in the know).  But if 25% of this event's attendance (a not so insignificant percentage) came from out of state, it shows some people would travel when and if able (and of course, there are business trips, vacations, etc. where it would be nice for people to be aware of Mass venues wherever life may take them).  
I reiterate: This post should in no way be seen as any kind of criticism of the bishops.  It is only a brainstorm trying to help the Resistance grow, and improve a less than ideal situation.  The bishops are our leaders, and what they say goes.  Would they be opposed to these ideas?

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Dr. White provided sufficient caution above to imply that perhaps + W asked for a higher degree of secrecy in this instance, for whatever reason. Perhaps, for example, he was given to know that a particular person or two of means was looking for the chance to disrupt his anniversary. There are many possibilities. +W has had to learn the hard way that his enemies are not averse to unusual degrees of intrigue in order to interfere with his plans or cause him to suffer. Our Lord said, be wise as serpents and simple as doves.