i personally am not a fan of modifying/messing with the bootloader too much,but i have someone willing to do to the edits who knows how to do them safely.
i dont have this device on any carrier,so im not familiar with current hboot version and build numbers.
we basically just need the following for each variant(sprint,vzw,GSM t6ul):
-a copy of hboot
-product id(aka t6ul or t6wlv)
if hboot is not available from an ruu or OTA packack you can dump it with:
su (if you need to to get a #)
dd if=/dev/block/mmcblk0p12 of=/sdcard/mmcblk0p12
upload the hboot image and post/pm a link along with the other info