As long as you keep the ambulance out of combat, it would do just fine for renforcment.
The micro for setting it up and keeping it out of combat is pretty much impossible in 1v1 unless most of your units are there instead of reinforcing soft-retreating squads while your forces are largely fighting like HTs can do, I find. If the lines in front of it give, amberlamps is in a lot of trouble. So basically, it only feels like a blob retreating to it and thus defending it (requiring a Major) has any real chance of reinforcing by the front.