I guess that means the new weapon UI (the deadline for which depends pretty much on Firebrand's time) has to be finalized for 4.0 instead of for 4.1, or can the unfinished new system co-exist with the legacy weapons? Or maybe we could use the KA weapon system for the interim?
Not sure what you mean when saying "KA weapon system". KA used same system as original Hexen.
To elaborate, there are 2 weapon systems (they differ both UI-wise and code-wise):
1. Original Hexen system, like any else game of that kind: a number of slots bound to certain keys (numbers by default), each slot may held a number of weapons, which types are, in general, not restricted (i.e. hypothetically there may be any types on same key). All weapons are freely chosen by pressing number keys.
2. New KRPG weapon system: a number of categories bound to certain keys (e.g. numbers), each category helds a number of weapons, restricted by type. Player sets up an active weapon for each category in special arsenal menu and then can only toggle between those pre-selected weapons (he needs to reopen arsenal menu to make different weapon set).
Codewise KRPG system is fully implemented; UI-wise it is implemented as a plain text menu (AFAIK).
And yes, it is absolutely possible to use legacy weapons with it (just needs little changes in code). In fact all original weapons are still in KRPG.