As I said, this isn't feasible for at least one of the PCs.

Mea culpa - I didn't read carefully enough.

It means that I now have to replicate 5 domains internally as well as externally.

How many A records do you have, and how often do they change? I run a pair of internal DNS servers with a few dozen internal A records (as well as caching everything else) for several hundred users inside our VPN. They use public DNS servers when they travel, with DHCP feeding them the appropriate servers. I have to edit my servers about once every month or two, and it's mostly painless: when I send a request off to the public (authoritative) DNS hosting service, I've (usually) already made the change internally for testing.

If I use a proper PC, could I configure it to do this properly?

I really don't know what router software might be able to do it, I'm afraid. All the "appliance" boxes I've worked with won't, and I really didn't want to custom build something.

-jk