I won't mention which particular ROM this is, as I am only using it for personal use and have no wish to make the developers of the particular ROM feel as though I am expecting support for this clearly unsupported endeavour. By copying over the appropriate libraries (APNs, RIL drivers, etc) and editing build.prop, I was successfully able to boot the ROM, and all network functions worked with the exception of receiving phone calls (mobile data of all speeds, outgoing calls, send and receive SMS, wifi, Bluetooth, and GPS). Would anyone here have any clue what I could have missed? I realize that this is a fairly open ended question, and am not expecting a lot.
Okay, here is a quick breakdown:
(The rom is question is based on the official Verizon 4.3 TW build)
1) I copied apns-conf.xml, GlobalConfig.jar, SecPhone.apk and more or less any library file with 'ril' in the title from the old rom to the new.
2) I edited build.prop to replace every reference to Verizon to reference T-Mobile, including device names
3) I did a factory reset (inc cache and Dalvik), installed the repackaged rom, then installed a T-Mobile 4.3 TW friendly kernel.
4) The default apn was epc.tmobile.com, which I had to change to fast.t-mobile.com to get LTE. After this change EVERYTHING net related with the exception of receiving phone calls worked.