you are uncertain about what the probability really is

I think this is a misunderstanding of the idea of probability. The real world is either one way or another, either we will actually win the sweepstakes or we won't. Probability comes into the picture in our heads, telling us how likely we think a certain outcome is, and how much we weight it when making decisions. As such, I don't think it makes sense to talk about having uncertainty about what a probability really is, except for the case of a lack of introspection.

Also, going back to Robby's post:

We don’t know enough about how cattle cognize, and about what kinds of cognition make things moral patients, to assign a less-than-1-in-20 subjective probability to ‘factory-farmed cattle undergo large quantities of something-morally-equivalent-to-suffering’.

This seems like an important difference to what you're talking about. In this case, the probabilities are bounded below by a not-ridiculously-small number, that (Robby claims) is high enough that we should not eat meat. If you grant that your probability does in fact obey such a bound, and that that bound suffices for the case for veg*nism, then I think the result follows, whether or not you call it a Pascal's mugging.