If people wish to discuss the attitude of the SSPX towards sedevacantists, this historic letter might be a good focal point.  Most of the Nine were sedevacantists.  They were concerned, amongst other things, that the leadership of the SSPX was at that point not permitting priests to hold the sedevacantist thesis.

From, "Letter of 'the Nine' to Abp. Marcel Lefebvre":

http://www.traditionalmass.org/articles/article.php?id=48&catname=12

   

Magisterial Authority

       The present situation in the Church has generated many unprecedented problems of a theological and practical nature — for example the question of the in se validity or invalidity of the New Mass, as opposed to the question of the attendance at the New Mass. On the one hand, the definitive resolution of speculative theological questions must await the restoration of normalcy in the Church. On the other hand, we must apply Catholic moral and dogmatic principles to practical problems.

      The Society must not presume to settle such speculative questions in an authoritative and definitive fashion, since it has absolutely no authority to do so. Any attempt by the Society to teach and impose its conclusions on matters of speculative theology as the only positions suitable for a Catholic to embrace is dangerous and opens the door to great evils — for it assumes a magisterial authority which belongs not to it but to the Church alone.

      Now while in theory the Society may deny any claim to such teaching authority, in practice it has acted as though it did have such an authority. For it has proposed solutions to speculative theological questions and has threatened with expulsion or has actually expelled priests and seminarians who disagree with its teaching.

      For example on Nov. 8, 1982 a young priest received the following ultimatum on the resolution of a matter of speculative theology [i.e. sedevacantism]:

If you remain with our Society, you have to gradually clarify your inner viewpoint and have to return to the attitude of the Priestly Society, which seems to us to be the only right one, under the given circuмstances, as a talk with theologians this past weekend has shown me again. Think about it seriously, because with this decision your temporal and so much more your eternal welfare is at stake to the highest degree. I will continue to pray for you for divine enlightenment and humble submission.

      Is this a threat of excommunication by a pope to a subject embracing heresy? Does not the prediction and threat of temporal and eternal ruination for a refusal to assent internally indicate the highest teaching and ruling authority?

      But alas this is not a pope speaking. These are the words instead of Father Franz Schmidberger, himself a young priest ordained in 1975 by Your Grace who will succeed you as head of the Society, and who presumes to teach and threaten with such authority. This is inadmissible!

      To act in such a way puts the Society in the dangerous position of assuming for itself rights and authority which belong to the Magisterium alone. It creates the potential for schism and worse. It is unacceptable from a Catholic point of' view. The Catholic thing to do would be for the Society to refrain from attempting to bind the consciences of its members on speculative theological questions which are, in fact, open to discussion, and which can only be settled definitively by legitimate authority when the traditions have been restored.

       

It would be helpful if each poster stated whether or not they agree with what is stated in this letter before commenting. If you don't agree with it, please say precisely what you don't agree with, and why.