For what it's worth, GL_ARB_texture_compression_bptc should be easy. Supporting the format for decompression should just be adding formats and passing them down to the hardware. You can look at DXT or FXT as an example. Supporting online compression is slightly trickier, but there are a variety of open source compressor libraries out there which already implement that. The project would basically be to evaluate those, and add code to Mesa to use that library.
I think it'd be a reasonable project for a new contributor.