I wouldn't necessarily agree that it is supported just because it is going via XOG. It may work, and if there's nothing actually wrong currently with any of your collaboration manager or project collaboration setup, other than wanting to set the PM to be the collaboration manager, then it probably will just fine.
It is however taking advantage of a situation that isn't a regular feature of the XOG user read/write (when you read a user out, those groups are not present). The risk is just low unless the following also applies.
There are times when the PM isn't the collaboration manager and neither is anyone else due to some other problem - and in those cases this method may cause further problems that we may not be able to help solve. Also if it ever does not work (just the same if the UI / url was being manipulated to access those groups which you cannot naturally access through the UI alone), we would likely have to respond to say that this isn't supported usage.