There were rules for customisation in one of the BattleTech rulebooks as well.....I forget which one. I think the one that had all the rules, from tier 1 to tier 3. But there were catches, there was the chance that your techs could screw it up, and you also had to 'balance' the design. You couldn't just rip out what was there and stick new stuff in, if you designed it wrong you'd incur penalties on your to hit roll and stuff. I'd have to track down the rules to be up on the details, but it was interesting.
As for bemoaning the lack of advanced gear, what about ComStar? They keep some very flashy kit around, though at this stage of the game they weren't too keen on showing it around, they mainly use it to make sure nobody messes with the HPG network.
There's also the possibility of an old Star League era weapond cache.....maybe there's a Highlander or 2 in there to do those DFA maneuvers (seriously, that's the reason the Highlander has such heavy leg armour. The engineers foresaw that with the introduction of jump jets, some hotshot, somewhere was eventually going to try and land on a mech, so they'd decided to make an assault mech that could stand up afterwards).