Matthew, you're on the right lines. The generic Chinese Li-ion blue battery has two leads, one which is the charging socket (female). That is permanently live and must NOT be interfered with by cutting into it. Putting it into a cab to act as charging lead is fine as is using it to be the feed for your receiver/controller, but of course you have to be able to get at the socket to detach the other wire when you want to charge the battery. The switch and charging socket approach outlined in this thread makes life easier!
The other wire on the blue battery is usually switched by the switch on the battery, but CHECK before cutting into it! As it comes with a male plug, it is very handy to use for the connection to the receiver/controller. As Matthew suggests, cutting the switched lead and spearating the wires is the approach, BUT remember:
1) only cut the male plug lead when you are sure it is not "live"
2) ensure that the cut ends of the wires coming out of the battery are separated and each is individually insulated with insulating tape