I think we have to make things a bit clearer:
1) You CAN flash your device with any localized rom WITHOUT having to hard-spl it (ROM must be signed). HardSPL is only needed to upgrade the radio part (and to block dangerous IPL/SPL upgrades too, obviously) and flashing unsigned cooked roms.
2) You prolly CAN flash your device without CustomRUU. Generally speaking all devices made by HTC can be flashed from the SD simply putting a .nbh file with the proper name for the device in the root dir of the SD expansion card. Then, pushing a combination of buttons, you can start the flash process.
For the HTC Panther (the codename of the TP), we can only guess what name we have to use (PANTimg.nbh and TPimg.nbh could be good candidates). Then we have to guess the right button combo. Hard? Uneasy? Let's try...