Darkplaces is great for playing Quake 1 with(to rediscover the game), and not much else, I'm afraid. The fact that its netcode is similar to QuakeWorld's(which catered to dialup latency) wouldn't help Tremulous, and would just get in the way. Q3's netcode is much better for broadband(it was designed for it) and dialup too, for that matter.
If Tremulous used XreaL it wouldn't even need to have re-done artwork. Who says we absolutely need awesome graphics right this second? The speed that XreaL can achieve with its behind-the-scenes rendering optimizations is worth it alone. After it's ported, later updates could include graphics enhancements. Plus, using XreaL would finally fix the shadowing problem with the current crappy stencil/projected shadows in the Q3 engine.