It seems Bambu Studio is missing the ability to mark an object as being support, which seems critical for multi-material printing. If I model a full contact (zero gap) support object, and set it to PLA beneath a PETG object, the slicer acts as if they will be one object and infills through the transition (as would be expected for a colour change). When the support is removed, the infill is exposed where there should be top/bottom shells.
Modelling a gap is not a solution, as that completely defeats the purpose of direct support from the dis-similar materials. I hoped setting an object as support enforcer and support mode to manual might have worked, but it doesn’t generate anything in that case.
It seems like the best solution would be the ability to set type as “support”.