The best success I've had with putting on weight (what is working currently) is just massive eating. It's going to be tough to fit into the confines of Paleo/Zone and accomplish that, but it can be done, just more slowly.
My opinion is that you need to ramp up the carbs to an extent. Fats are calorie dense, but at the same time they don't digest quickly and make it harder to eat a sufficient quantity of food. I would say that should also have the ancillary benefit of resolving the issue reported herein as well.
Just a thought - this may not be an attractive option for you.
EDIT: I x-posted with you and Dr. G. If you are careful w/ your calorie intake and carbs, you can gain mass w/o significant fat. You just have to ramp up the cals / carbs gradually until you start to put on mass w/o appreciable fat gain.
"Use what talents you possess: the woods would be very silent if no birds sang there except those that sang best." —Henry Van Dyke