Actually, Trellis is really similar to the ticket system GlowHost uses (ProSupport) -- so much so that I have been wondering if they share a codebase. If not, I'm pretty sure
Trellis's front-end and function logic are ripped off of it. Includes pre-seletion of "departments" before submitting a ticket, KB integration and search. But it also has email piping and several other great features. Also the PHP source code is among the cleanest I've ever seen -- I'm not much for PHP, but even I'm having no problem making modifications.

Anyway, I suspect it might not be impossible to replace ProSupport with Trellis in due time. The back ends are likely very similar.