Could we use an SRV to help an overladen cargo ship escape orbit?

Absolutely. That was one of the use cases that we had when we were coming up with this ship: what are all these scenarios in the game that people are struggling with? Ships running out of fuel on the surface of planets and having no way to get back anywhere. Reclaimers upside down, and other big ships like the Starfarer and Reclaimer struggling to get out of atmosphere, because whilst they do have VTOL thrusters to some extent, it's still a lot of mass and a dense, heavy atmosphere to get out of. Ships like this, that's great.