I don't think the idea is unreasonable. It says a bit about the issues that Pan has, though. Elsewhere, portal vaults are announced to make players change their approach to exploring the level (or not -- but it's a choice). Permanent portals don't need an announcement since we can expect players to fully explore the level anyway.
I would support a patch if it came in, but this is just a personal opinion. No idea if others would be resistant -- perhaps just ask on ##crawl-dev?
Probably a whole lot could be done about Pan, but I've never really been there, so ziggurats may be my only contribution ever to that branch