My thought was to ‘just add’ the static baked lighting to the final result, after all the lumen calculations are done. Obviously you don’t want the baked lighting taken into account for the lumen bounces since it already ‘bounced’ in the static calculation.
Yes of course, with both systems using VRAM simultaneously there will be a hard limit. You seem in a much better position to judge if that would make the feature unusable than me 
I would say the advantages can be very significant (from the perspective of designing environments, that is): When scenes are lit with many, many static lights (like a building with many rooms viewed from the outside or a street filled with streetlights), you become very constrained when fully dynamic lighting is required. In that scenario, using Lumen is effectively ruled out.
In the City Sample, for example, this is solved by completely removing streetlights and using only POM for inside buildings (so POM sort of becomes a substitute for static lighting). The moment you would want the option to actually go inside of a building in a scene like that, you would need more (dynamic) lights and you run in to issues.