I like the OP's idea. It's simple and it fills a small-but-critical connectivity gap.

Since Frontier's service out of TTN is (for now) limited, you'd really only have a handful of SEPTA trains you'd have to meet. Personally, I'd dispense with the reservations and simply make it a scheduled service for a flat per-person fee. Frontier (or, ideally, the airport) guarantees the shuttle operator a minimum amount for each run. If cash fares exceed that, then Frontier or the airport is off the hook. The reason I think the airport should provide the revenue guarantee is that it would help ameliorate their parking shortage.