I can't seem to get mattes to work with Redshift in Solaris. I have tried enabling the matte option both at the object level and in the Solaris scene graph with the Render Geometry Settings node. In both cases, the geometry is rendered as usual and not matted out. What am I doing wrong?

It's not very intuitive, but simple if you know how. You create a Render Var like you propably already did for the other AOVs. But since the cryptomatte is output directly by the Redshift render engine, it will need to go into a separate file, which you specify in the Render Var node under the Redshift tab, in the Cryptomatte section. You have to set or create the attributes there, and specify an output file. Then it will work fine.