I think the OP is a great question.
One might pose a similar question, as to whether or not a protestant or Orthodox Christian could make it to Heaven if he was martyred for believing in Jesus Christ.
The difference here is that a NO Catholic (differentiated from a plain liberal, as Anthony Benedict rightfully pointed out earlier) thinks he believes the true faith. In some aspects he probably does. And even greater a difference, he thinks he believes the true faith because the men that the VAST (we're talking like a billion people here) majority of self labeled Catholics (whether they are or aren't is another matter) reinforce this belief, and some of them even do so in good will. This is, of course, diabolical disorientation, but the waters remain muddied nevertheless.
I tend to think that among good intentioned Novus Ordites, we will see a great manifestation of God's mercy come Judgement Day.
I don't think there's any simple blanket answer to the question. It isn't comparable to a protestant or Orthodox 'martyrdom' because the NO Catholic actually thinks and believes that he is holding the Catholic faith. An Orthodox or prot may believe that he is holding the TRUE faith (though he is obviously not) but still rejects (explicitly) the Catholic faith, and would not admit to being a Catholic. A NO Catholic believes he has the TRUE faith (which is the Catholic faith) and recognizes that the only true faith IS the Catholic faith-- his error is an intellectual one.
Again, this is a complicated issue.