Maybe this are stupid questions and it would for performance reasons not make much sense.
Is the new cube map mentioned in readbeta dynamically updated (rendering into each cube face per frame) as in the DirectX cube map sample? If not, is it possible getting a “bmap_for_cubeface” function, which provides a bmap pointer to the individual cube faces to use it as render target?

Not yet, however it's on my list.

Updating a cube map is indeed only useful in very special cases. It reduces the performance by 85%(!), so if your game ran at 70 fps before, it will go down to 10..15 fps when updating a cubemap.