Any modifications to the base code are completely unsupported by UBBCentral // UBBSystems, and generally we recommend modification postings over at the
UBBDev the official development site for UBB.threads. That being said...
All of the threaded mode code should be handled by the template:
showflat.tpl - see {* THREADED MODE START *}
All of the Post Display data will be in templates:
post_side.tpl
post_top.tpl
I'll forward your idea over to the potential future features list for upcoming releases since it makes total sense to be an option for threaded mode.