Tubs

Dugast tubs (cotton Pistas in particular) have removable valve cores?

I imagine so-- at least the current incarnation. There are, of course, many valve extenders that don't need to have the core removed-- I'm assuming you want to use the tyre on a deep section wheel (the standard 38mm valves are, of course, too short) and not use sealant (something that you really never want to use on the track)..