Each "FrankenMech" is going to be unique, so having a TRO entry for a one-of-a-kind item seems like a waste of space. I'm pretty sure that such a piece of equipment is going to have quirks, technical issues, possible balance or targeting issues, and other reasons why it's generally avoided except in odd circumstances when nothing else functional is available, but you've got half of "this" and the other half of "that" on hand, and someone willing to suffer through the process or pay to have it done, regardless of the outcome.
They're clearly not "tournament legal", and I typically don't use them (except in one weird campaign where the players faced such a Medium machine with only a couple of beat-up Light 'Mechs and vehicles - as a way of providing them with some badly-needed salvage, since any decent Medium would have torn them apart at that point). Any such design should be intentionally riddled with bugs and design flaws, penalties to various odd actions, and other things to clearly indicate that it's a cobbled-together mess, not a properly designed machine.
