Hi Dan_Hat
There are two driver commands on the DLS by elstoko, "Set Signal Ahead" and "Set Selected Signal State v4" which allow you to force a signal to a particular state. "Set Signal Ahead" will allow you to set the next signal ahead while "Set Selected Signal State" needs the signal to be named and is used where the signal to be controlled isn't the next one in front of the loco.
If used with the basic Auran UK signals with calling on lights fitted below the main head, the main light will stay red but the calling on lights will illuminate until the loco has passed the signal.
These two driver commands are perfect for your situation as I use them to allow locos to back down onto trains at King's Cross on the ECML route.
Regards
Brian