it could probably be added but would break protocol with regular quake engines my guess is thats the reason why there has been some hesitation to do it.
i.e it would create more problems since not all use the same engine of choice

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this was a reasonable middleway so to speak.
if you dont care about compatibility i can provide some work i done on the last tenebrae2 engine which uses Quake 3 map format and uses shaders for realtime lighting (can be added with most good Q3 map editors) the engine is far from done though but if someone picks up where i left it i think it could reach a usefull state for a custom engine pretty fast as the things remaining to fix are quite small.
most notable buggers it still has are weapon poking into the walls
animated models (roq format) has no collision detection and shadows sometimes cut off to soon.
minor bugs in the menu system which uses xml (flickers a bit) but is mouse driven and works a bit like doom3's.
its pretty fast on modern hardware.
i can link a copy with sourcecode sdk and a miniversion of industry (tenebrae mod) so you can have a look at what its capable of.