Tremblant is a regular trip for us, usually the first week of April. But it's a drive-to ski destination for us and we wait until about 3 or 4 days before going to commit to the trip. Most years it's good to very good, but some years it's a bit spotty and sometimes we don't go at all due to premature spring.
Temblant offers a spring pass which gives unlimited days ~ mid-march to close for about $225. This should give you some idea of the level of crowds - if it was crowded, they wouldn't be offering a deal this good. Nearby lodging also goes for cheap and since the place is almost empty you can wait until the last minute.
Tremblant is probably as good a bet as any for east coast late season. If Tremblant is experiencing an early thaw the others will probably be in a similar situation. Conditions will vary, as noted above. I think it's a pretty strong bet that they will have something open to ski on, but it may be just the upper half of the hill in a bad year. Of you may get lucky and get there just after a two foot dump like we experienced one year.
The last couple of seasons we've caught the last day of La Soleil and The Edge before those two areas closed for the season. But we've been going a week of two later in the season than your plan so it's likely that the whole place will be open.
Bottom line is that I don't know that I'd fly across the ocean for an east coast trip that late in the season. It will most likely pan out, but there's a chance it won't.