I do not see why the comstar/star league tech should not be available. While it may be rare or expensive at this time, it did exist in some form. So I say make it available, but make it expensive to buy and maintain. So while you can pilot the standard mechs for much less, if you are willing to pony up the dough you can use more expensive mechs.
Maybe as a temporary power-up method. In-universe, your pilot rents a powerful Lostech/Star League weapon or sensor suite for a mission, paying a cost in C-Bills. It gets brought back, you lose the power-up (so they can repair and maintain it), but get a portion of the cost back as a deposit. If it's damaged or destoyed, you don't get the deposit back.