TUAW to the rescue. After polling the developer community, ApolloIM developer Alex "Arminius" Schaefer came to the rescue. He remembered that SummerBoard supported a mode that would retain your signal even in sleep mode.
The hack works like this: You install SummerBoard (it is available via Installer.app) and then create a temporary file called SummerBoard.DisablePowerManagement. Once created, SummerBoard sees this file and keeps the signal from powering down.
With this knowledge in hand, I was able to create a small utility for John, which is hosted here. To use, just tap it each time you reboot your iPhone. It creates the temporary file, which lasts until your next reboot. (All tmp files are deleted then.)
The JohnTool application will be available in short order via Installer.app and is already available at the PXL package repository.