Tasks must match geography... For both Reg and Res.
Weekend exercises can be fun with Reg F units. When the 2Lt duty officer reports on Monday morning that the Res unit brought out their Honorary Colonel, and that the retired LGen, former Army Commander, had a great time, the email chains...
The challenge with the CDS is not "who can charge" but "who can preside". Any charge giving rise to a GCM would be impossible, as it would be impossible to form a panel.
For a military judge only trial where there's no right to elect a panel, on paper, the NDA would permit it.