I have an 8'x14'(outside dimensions) mudroom attached to the back of my house with 2"x6"x14' ceiling joists/rafter ties @16" centers. I have 1"x6" collar ties installed and installed a center support (L-shaped) made up of 2"x4"'s running perpendicular to the ceiling joists which I tied one end to the gable end of the outer (14') wall of the mudroom and the other end to the existing house roof sheathing. I installed a 2"x4" vertical support in two spots which tie to the ridge board on one end and to the 2"x4" L-shaped support on the other end which is tied to the top of each 2"x6" ceiling joist. I have installed 1/2"x4'x8' drywall to the ceiling joists and would like to know if it would be safe to now install over the 1/2" drywall 3/4"x6" T&G wood planking.
Yes, that should be fine - not that you needed the drywall, but no point in taking it down since you've put it up. Just account for the thickness of the drywall when choosing the length of fasteners for the T&G boards.
See here for direct application of planks to ceiling drywall.