Since commit 497, yes !
I've just added a new function (must be called before any texture loading, obviously) : "raydium_texture_compression(1)".
It's also possible to enable this with the "--compress" command line switch.
Compression level is somewhere between 2x and 4x, and I've a better framerate with this feature enabled ! (197 FPS vs. 206)
I can't enable this feature by default in the engine, since compression generates some very bad artefacts on normal maps:
edit: compression choice is left to OpenGL driver, but the way Raydium ask it makes the driver to use almost eveytime S3TC (DXT5, I think).