I tried painting epoxy upside down under the car today

Had a hard time manuvering the gravity-feed gun I have. I guess it's better to use the type of gun that has the cup on the bottom?

Have you looked in the mirror yet?

Or you can use 3m's flexible liner/cup system. I forget what its called. THey came out with it about 7-8 years ago. It works pretty well until the paint level gets low, then the bag can inhibit flow.

Siphon gun as you have figured out is an option, so is a swivel fitting on your air line, and raising the car as much as possible.

You can buy remote pot spray guns too.

Use a siphon fed gun (cup on the bottom)and turn the pickup tube around so it's facing backwards.Use a cheapo hood/facemask from Home Depot, and wear goggles, cause you WILL get covered up with paint!