From Googling and forum-diving it seems that the way to get a raw diffuse AOV from Karma is to "Use CDA as a light path expression", but I haven't been able to find out what exactly that means/where to do it. I know what an LPE is in the most general sense, but haven't found a field in a node where typing "CDA" does anything. Any pointers?

On Karma Render Settings under Image Output > AOVs

Cool, thanks! This is not raw, though (I might be using the term incorrectly, from my fuzzy old mentalray recollections)--the lighting is still represented. Is that just a relic of old rendering that is no longer possible?

Sounds like you're wanting to export the diffuse textures or colors to an AOV rather than the albedo. You can do this at the shader level with the KarmaAOV node.
https://www.sidefx.com/docs/houdini/solaris/karma_aovs.html [www.sidefx.com]

Exactly what I was looking for. Thanks!