There is no such thing as a 'concave' or 'convex' geometry for a BSP tree. A BSP tree is plane based.

The limitation to convex blocks in most BSP-based engines, like A4 through A6, had other reasons. Both the light map compiler and the PVS system relied on convex blocks. That's why the A7 map compiler needs more time for lightmapping when blocks are not convex - we're using a different algorithm for such blocks, which is slower.