I found a method (kind of by accident) for preconfiguring UBNT radios before Elevating them, and thought I'd share, in case this is useful to anyone else. It turns out that this is already possible with the current ePMP elevate firmware (I tested it on 3.2.2).
I haven't tested this on XM devices yet (I assume it'll work the same), but I did just test it on a couple of XW radios (PowerBeam M5 and NanoBeam M5), and it appears to work perfectly... of course it's possible that will be issues with this method in some cases, which I haven't run into, so if you brick a bunch of radios, don't blame me ;)
The process is as follows:
1) Elevate an UBNT radio on the bench (you're going to want physical access to it)
2) once it's up and running with the ePMP elevate firmware, configure with all the necessary settings for a generic template that will be able to connect to your network, and save the settings (you could do this for each individual customer, if you have settings like PPPoE that are unique for every customer, but that would be very time consuming and doesn't make sense to me... you could probably also manually edit the config files for each customer, but I don't want to get into that - using a generic template seems simpler to me).
3) downgrade to UBNT firmware through the ePMP web interface (do not use the tftp method to get back to UBNT firmware, as that would erase everything).
4) Use an SCP client to copy the entire "/etc/persistent/mnt" directory from the radio
5) copy the directory "/etc/persistent/mnt" directory that you save from the pre-configured radio to another UBNT radio (you may need to use a UBNT firmware that allows custom scripts for this to work, like 5.6.4 or older... I haven't check that yet).
6) login to the radio you just copied the files to with SSH and run the "save" command
7) Elevate radio
After the upgrade process is complete, it should come up with all of the settings you configured under the ePMP elevate firmware on the first radio. It doesn't appear to matter how the second UBNT radio is configured, it looks like the ePMP firmware won't try to copy any of the settings it normally would if it finds an existing configuration.