FWIW, I'm not "winging it" and getting away with it. I worked on some very high end electronics systems in the Navy and I've been flying RC since my first Cox 2 channel glider radio and have a lot of experience with different facets of RC including gas planes. I don't mount a reciever to an ignition box but I do install my components where they make sense and are convenient without regard to the 12" rule, simply because it doesn't matter and many installations don't allow you to obtain that. Heck I power my 50cc stuff with one A123 battery that runs the Rx and ignition. I have hundreds of flights in this configuration, it works. Bigger than 50cc I run 2 A123's to a Power Safe reciever, one battery is split to the ignition.

I've never seen an RF problem that I have had to move components (or one of my favorites, twist servo extensions) to fix.