Just had another reply from the technical department at MA. They indicate that in-wall speakers without a supplied back box (eg CT265) can be placed into a box, allowing on-wall installation. As a rule of thumb, they recommend a box volume of 13 litres. Interestingly, they state this is the best volume for all of their custom installation speakers.
For me, this suggests that you should aim for a void of around 13 litres behind MA speakers when installing them in-wall (assuming they are not from the Controlled Performance range).
For the avoidance of doubt, here's their reply to my question regarding suitable speakers to partner with the CP WT380-IDC fronts.
Hi PJPro
Another good thing about our custom installation products is that they can be mixed and matched to a certain extent. If you weren’t worried about having a back box on the rear speakers (the back box will reduce the sound traveling into the adjoining room by about 30dB) then you can use the CT265 if you wanted to. If going for a 7.1 system, you could also consider the CT265-FX.
You can make a back box for the normal CI speakers if you want. The best volume to aim for is around 13 litres and this applies to them all.
I hope this helps?
Best Regards
Technical Services Department