Valve's self-shadowing radiosity normal maps concept can be used with spherical harmonics in approximately the same way: Integrate a sphere based on how much light will affect a sample if incoming from numerous sample direction, accounting for collision with other samples due to elevation.You can store this as three DXT1 textures, though you can improve quality by packing channels with similar spatial coherence. Coefficients 0, 2, and 6 in particular tend to pack well, since they're all dominated primarily by directions aimed perpendicular to the texture.
I use the following packing:
Texture 1: Coefs 0, 2, 6
Texture 2: Coefs 1, 4, 5
Texture 3: Coefs 3, 7, 8
You can reference an early post on this blog for code on how to rotate a SH vector by a matrix, in turn allowing you to get it into texture space. Once you've done that, simply multiply each SH coefficient from the self-shadowing map by the SH coefficients created from your light source (also covered on the previous post) and add together.
