Yes, but I'm not sure that we should allow parameterized extensions without a where clause. That's not necessarily a constraint on expressivity: we could allow an otherwise-empty where clause, used only to introduce parameters. That would at least put the syntactic emphasis in the right place at the cost of writing one additional keyword. I think conditional constraints are rare enough that it's probably a good trade-off.
P.S. I took your hint and posted in the other thread, in case you'd prefer to discuss it over there.