I hope it's okay to wake up such an old thread. This is something that interests me, because I like micro RC models. I joined the forum just to contribute to this topic. It looks like a very interesting forum though, so I'll be back
Some servo boards/amps make quite good ESCs for cars or boats, provided the motor is smaller than the one that was in the servo - they're not normally intended to be moving continuously, so it can't necessarily drive the original motor for long, unless you're careful with the gearing, to keep the normal load quite low.
Some servo boards actually make very good ESCs, with a degree of proportionality - often enough to be useful, for a small car or boat, and not just full forward / full reverse of off. It depends on the design of the servo. Some respond to being further out of position by moving the motor quicker (that's the best sort to use), some just always move the motor at full speed (not so useful - those just act like switches).
I turned a twin motor toy boat into a proper R/C boat using the boards out of two old Futaba full sized servos. That works well, and the boards made very good proportional ESCs.
I like the idea of adding a bridge chip to boost the current output. I can see how it might turn a proportional output into a switched one, if the output of the servo board is linear, i.e. it varies a smooth current. There's a good chance that the output is PWM though, in which case it will work just fine.
I may have to do some experimenting
I found a source of 5A simple single cell ESCs with reverse, recently (banggood.com), but they stopped selling them, and deleted the product page, so I may have to make my own.
There are plenty of small ESCs with reverse for 4xNiMH cells, or 2s lipos, but none available that I know of, for single cell lipos, or less than 4 NiMH cells.
One option is to use a 2.4GHz DSM/DSM2 DelTang receiver, which has ESCs built in - there are various models, with different numbers of ESCs. For very small models, that's a good way to go, if not a particularly cheap one. He sells add on H bridge boards, called ADD1 (800mA) and ADD2 (3A), that can be connected to them to boost the outputs. Those are quite inexpensive, for what they are.
See deltang.co.uk and deltang.co.uk/addon.htm (sorry, I'm not allowed to post them as links, yet).