It seems like 'combinatorial explosion' might not be the right phrase here. There is a combinatorial explosion of possible prefixes, but, since you're not going to search the possible-prefix space, it doesn't really matter. (For example, there is a combinatorial explosion of unprefixed variable names, and nobody's bothered by that!) The question is whether you can decode a given prefix, and, even in your example, that's easy. (Prefix ambiguities could easily be resolved with underscores, like url_html_sql_b64_string.)

I'm not arguing for this convention (I don't much like it myself); but I don't think your argument is a very strong point against it.

EDIT: Oops, sorry, I just noticed that it wasn't you who brought up the explosion in the first place.

uh, why would you have one of those except to crap on the idea? have you ever had a sql and html escaped string?

No, but I have had b64 urlencoded htmlencoded strings.