I can't get command buffers to use shadow maps

I'm writing a screen-space effect shader using the new command buffers in Unity 5, and I need this shader to access the scene's shadow maps. I'm following along with Joey Fladderak's guide on shadow effects here: http://gamasutra.com/blogs/JoeyFladderak/20140416/215612/Let_there_be_shadow.php and it seems to work fine in general rendering cases, but it does not choose the shader variant I want when rendering from a command buffer. It always uses the shader variant with the SHADOWS_OFF keyword, even if I disable that particular keyword. Help?