Like I said, it's not really an "extensive" change. Well with Quake3's bundling it might turn into one, but I think for the most part pass accumulation is done in realtime. IIRC they're not even using the bundling "optimization" by default.
All that it would require is rendering the lightmap first, dynamic light attenuation second, and the texture last. If you add 3D texture dlights (look at EGL or QFusion for an example) you'll have perfectly spherical dlights. Right now Quake3 just renders the dynamic lights right on top, which isn't proper modulation and is why the lighting is multiplicative.
Hell you even open the opportunity for dynamic light shaders. The dynamic light from the lucifer cannon can have a slight "star" shape with noise, spin, etc.
Even if realtime lighting is added, this doesn't meen you necessarily have to add the stencil shadowing (which is the slowest part to "true" realtime lighting). You can use it in conjunction with lightmaps. Take a look at HL2 or Oblivion, they do the same -- only player models cast stencil (or shadow map, in the case of Oblivion) shadows. With just simple DOT3 bumpmapping you're not going to see a significant framerate drop, especially on cards that don't software emulate the 3rd+ texture units. I don't think it would be a wise route though, just because this is Quake3 so not everybody has a GF4+.
