I seem to recall the Carriers having some pretty strict requirements on the devices that can connect to the mobile networks. Anyone know if that's (still) the case?

I'm not trying to defend Apple here, I'm just curious if there would be some kind of carrier validation issues if you slapped a full desktop OS on a phone.

You can connect to 4G with your root-enabled Linux PC and a USB dongle or minipci module. Carriers don't care about your application processor, they only care about the baseband. In the case of a smartphone, you can have root access and still run the Qualcomm closed blob firmware that will drive the baseband

I doubt that's the issue. Phones already have a baseband processor and OS in control of the modem. Also evidence if viability is all the Windows laptops with WWAN.