We do it this way: 3d modeler creates a geometry and then assemples diffuse and lightmap separately. Both UV channels are saved with geometry in f3d, along with diffuse map. Then, in AS3, I load f3d and load separate lightmap png image, add it as a texture filter with a second UV channel, blend it as "multiply" and it works like a charm.
Anyway, we are looking forward to playing with the 2.5!