Why not just use BC7 (BC6H for HDR) textures on desktop and ASTC on mobile? There's no need to ship an format like basisu that converts to both. There are rust bindings for both the Intel ispc BC(n) compressor and ASTCenc.
Oh but if you care about GPU support then I'm pretty sure https://github.com/GPUOpen-Tools/Compressonator has GPU support for BC(n) compression at least. Could rip those shaders out.
Basis Universal also gives you much smaller size over the wire than hardware compressed formats (closer to jpg), which is important at least for web games (or any game that streams asset data over the net).