When you switch to A/G mode, have no weapons selected, and set your HUD as SOI, you can use WEAPONS RELEASE to designate a target using your hud (can be slewed with the TDC).
However, when you select and then deselect an air to ground weapon in the stores page you can never designate another target again that way.
Track attached (after selecting and then deselecting one of the bombs, I hit Weapons Release again but can't get the target designation to work).
Looks like an edge case, but I would love to hear more from the devs. HUD Target designation not working.trk