If you have access to a lathe, the following is a slick method:
1. Get an AN832 bulkhead fitting in the correct size for your hose, and a 1/4" push-to-connect air fitting with a 10-32 threaded end.
2. Cut off one end of the AN fitting and machine it flat. Drill and tap the hole for 10-32 threads.
3. Thread the air fitting into the tapped hole. Seal the threads with Permatex or your favorite sealant.
4. Install the AN fitting on the firewall with the usual AN924 nut, and washers as required.
5. Attach your flare hose to one end, and your 1/4" nylon line to the other.
Voila, custom bulkhead fitting that's also an adapter to go from AN flare hose to 1/4" plastic tubing.
The following is not quite the same configuration (different plumbing arrangement in this case) but the idea is the same:
Note: I wouldn't use this for anything with actual fluid flow - that fitting has a little tiny orifice - but for manifold pressure it's perfectly adequate.