I have never done a butt in a pellet cooker. I use my little cookshack electric, it just works far to easy with the results we like. That said it has a hole in the bottom I slide a cake pan under it catches all of the drippings. I usually boil mine after I remove the fat, just because it is out side of the smoker and temp wise I never know what it is but usually the fat is already hardened up. The stock has a deep smoke flavor. a huge difference in flavor between adding some back in and not.
I think your plan is great and you will be very happy with the result.
When I say easy, put a few chunks in the wood box, foil the top of the box and bottom of the cooker put meat in, turn it on and come back in 12 hours to take it out and rest in a covered meat lug for a few hours.