I think something like that would require a lot of changes and probably something that wouldn't happen any time soon. 3G data, 1x data and text messages all currently are part of the CDMA tech. Also, currently roaming only works on 1x CDMA on Verizon towers, so if you didn't have a Sprint LTE signal, you'd have no service at all as the 2 LTE technologies are different and not compatible with each other.
Seems like another reason to avoid Sprint and their version of GNex. The battery life on the Sprint Version is significantly less than the GSM GNex from Google running on T-Mobile. The Sprint was down to 47% just receiving mail and 25 minutes of calls in only 5 hours while the GSM version was only to 87% running hotspot all morning and surfing the Internet and posting. The T-Mobile version regularly lasts all day. I have to keep the Sprint version near an AC Adapter. It looks like the weak Sprint network causes the phone to search constantly for a signal. It also runs significantly hotter than the GSM version also most likely related to turning up the juice to try and maintain a signal.
I hope LTE next week really boosts performance.