err yes you can use lipo's with brushed motors so long as the esc is lipo compatable ( whether you use a brushed esc with a low violtage alarm or a or low volt cutoff )
It is my understanding, like Stormbringer, that Lipos ,like any other battery, supplies dc current at its nominal voltage which the 700BB is very happy with, (and will work quite a bit above its rated voltage as well). The same is basically true with many ESC's , they are only controlling incoming dc current within their voltage range, irrespective of the type of cell supplying it.
Where you have to be careful is that with the high energy density of LIPO's compared to NIMH or nicad cells, they can supply that dc voltage at high amps so the motor and ESC have to be able to cope with the load applied to it. By that, I mean, that just because a LIPO
can discharge at high amps, it doesn't mean that it just will. It should only supply the level of power that the motor demands of it depending on prop size etc. A simple safety measure when using LIPOs in any i installation, brushed or brushless, is the inclusion of an inline fuse of appropriate rating in the wiring from the battery to the ESC
Probably the biggest issue with LIPOs is running them down below a certain voltage which will ruin them . Again, note "certain voltage" as people will argue till the cows come home on the various forums (fora?) just what that value should be. Like Stormbringer said, most lipo compatible ESCs will either have a suitable set value or a choice of several values, which when reached will either cut the supply to the motor or reduce it to enough to get the boat to shore.
If using a brushed esc then a simple and cheap alarm can be used that plugs into the balance plug of the lipo, which when the low limit is reached will sound a very loud and obvious alarm. If you don't take notice and end up with knackered cells, don't say that you weren't warned.
Lipos can breath new life into old models, especially those who were tight for space for nimh packs as there can be a considerable size advantage.
The only other issue is to get used to the slightly more stringent charging routine which you need to use for lipo cells