I'd say there's probably not a place for Tarsnap at YC (though I'm not sama), there's simply not enough ambition, but I'd be surprised if there isn't a place for Colin Percival. There obviously already is at HN and I bet there's dozens of startups that could use your help or advice in some way.
You mentioned a lot of reasons why Tarsnap can't be in the YC community. Perhaps you could come up with some reasons why you would have a place? It's marketing man ;)
Well, it's hard to separate "Colin Percival" from "Tarsnap". I mean, yes, what I really mean is a way for me to be involved; but I have every intention of continuing to run Tarsnap for the foreseeable future, so they can't really have me without also having Tarsnap.
You mentioned a lot of reasons why Tarsnap can't be in the YC community. Perhaps you could come up with some reasons why you would have a place? It's marketing man ;)