ORIGINAL: earlwb
... They do have a special BEC device that can use a single battery pack to power both the ignition system and RC system. it uses RFI filters and adjustable voltage regulators. Here is one such device
http://www.tech-aero.net/ultra-ibec
...
Actually the url is
http://www.tech-aero.net/ultra-ibec (without the extra spaces at the end). Looks like a decent product, but I have no experience with it.