It's not impossible that God might respond equally to anyone who fulfills a certain list of criteria, regardless of what religion the person follows. A devout Buddhist may have as much chance of leaving an incorrupt corpse as a devout Catholic.

On the other hand, it's also quite possible that the phenomenon of incorrupt corpses occurs regardless of the virtues of the individuals in question, but then corpses of the particularly virtuous are held up as examples of divine grace, while the incorrupt corpses of ordinary people, not being seen as evidence of anything in particular, are ignored.

You mentioned before the possibility of militant atheists cherrypicking evidence to support their position. This is certainly a consideration that has to be accounted for, but so is the possibility that the evidence favoring religion only appears compelling because it is cherrypicked. This also occurs to a considerable extent with nigh-certainty. Consider, for example, the healing miracles of Lourdes, which Nornagest mentioned above, which have made it an international pilgrimage destination, despite the fact that statistical analyses of the recovery rates of pilgrims do not suggest that the location has any particular healing power. Counting every unexplained recovery, while not counting the nonrecoveries, can create the appearance of persistent miracles.

Hard-to-explain things happen all the time, and we're much more likely to notice them if they seem indicative of something important to us than if they don't.