Transclucent objects baking AO, as if they were opaque?

For some pieces of world geometry, I have "decals" that are being applied to the terrain - basically semi-transparent (achieved through vertex alpha blending) objects that are overlaid on the base geometry to simulate blending of textures:

Bare:


Combined:


This works great for its intended purpose, and fits the look I was going for, in realtime lighting.
The problem now is, in wanting to add some ambient occlusion to the scene and baking the lighting - these decals are being treated as if they were on large opaque quad:



Just wondering if there's any workaround for this? My lightmapper settings are as follows:

 

Thanks for answering!

I stumbled upon this in the end.
I don't suppose it's possible for a static object to "receive GI" only, without contributing it? (Specifically for ambient occlusion, which I don't believe light probes provide?).