Can I permanently disable the cell/radio of my phone?

My wife got a new phone with a new service, so I get to have her old phone to play around with. Since the old phone no longer has cell phone service, I want to completely disable the cell phone or radio portion of the phone so that it doesn't eat up battery. This is a CDMA phone, so there's no SIM card I can just take out. Enabling Airplane mode works just fine, except that also turns of WIFI, which I have to manually turn on. And if I reboot, the phone stays in Airplane mode, but it turns WIFI back off. Is there a way to modify the phone to turn off the cell portion completely and permanently, at least to be able to withstand a reboot (not necessarily a factory reset)?