I tested & berserker isn't compatible with local old Q2 mods but it can connect to r1q2 dedicated servers running old mods (IE the Q2 SDK).
Now this data could be collected, for each server and compiled into some sort of neat format or in game server browsing tool. Like I said, it's messy, but it's something to start from. I'm currently thinking of ways to implement it.
So it can't act as a server for non-berserker clients, but can be a client for old servers instead. I wonder if any of the better quakers from TS community use berserker. But probably it doesn't even support Anti-Cheat? But the big plus is, its under active development (other than other engine mods).
But, from what Paril told me about CleanCode, as long as the dll is compatible with the engine it should work. Both CleanCode & Berserker are C++ vs C.