> How long do you think defcall and settable-fn would take?
About a week, maybe less ^^. Defcall is reasonably trivial although it requires some rethinking. settable-fn is implementable using defcall (see nex-3's settable-fn2) but I'm personally dubious of such a style, and prefer my own.
> (But I'm a little dubious about working on something pg may have already done.)
We don't have evidence either way - pg hasn't commented on this (none that I've seen, anyway). Up to you to decide whether to act as if pg made it, or act as if pg didn't make it.