In LOMAC, on ground attack missions, one can see preset targets on the HUD and MFD. If these targets are moving, the diamond markers follow them. How does this work and is it realistic?

I think the theory was that there were ground troops needing the support, so they are essentially marking the targets for you with a laser.

As to how realistic that is, well, I've never flown an A-10 or Su-25 on a ground attack mission, so I can't say for certain, but I have read where A-10s have a sensor mounted just below the cockpit that detects the laser energy of a target being painted, so I believe that it could display a marker on the HUD.

Since it never carried its own designator until the C model, having it marked by ground troops (or maybe another aircraft) would have been the only way for that detector to be of any use.

Anyone can, and of course will, correct me if I'm incorrect on anything here :-).

The A-10 is equipped with the Pave Penny pod, which is a passive laser sensor that can detect and display spots that are lased by another source (aircraft or typically ground units). This is what is replicated in Lock On, although obviously in a simplified way - there are no actual ground forces around, and the laser is on the target all the time.

In the mission editor this can be achieved by assigning targets to the attack waypoints, they will then be available as "laser targets".