In terms of 'seasoning'... Not sure I'd do anything. It's all stainless, so rust isn't a factor really. Also, if you ever do really high temperature cooks , it's going to pretty much burn any seasoning off. Not that it will hurt anything to season it, I just don't really see a large need. I certainly wouldn't get very concerned about it.
I never really considered foiling my ash tray , or drip tray. For the ash tray, if you do line it with foil, I'd just be careful about not letting the foil stick up where it might catch on something as you pull the ash tray out, if it were to catch on something, it might make more of a mess than you would like and would totally defeat any possible advantage there might have been in putting the foil down.
I don't use my warming tray much , in fact I'm not sure when the last time I used it was. That area gets lots of smoke, and mine is almost certainly a permanent tinge of smoke colored staining. If I did plan on using this much, I might really consider this being a primary spot to foil line... but again, if I did foil this, I'd be careful about how well it slide in and out with the foil on. The reason I might consider foiling this area, is to try and keep the smoke staining on the inside of the drawer to a minimum.
My PG500 is inside my shed, so I never have had it covered, no need to.
I don't really try to keep my PG500 spotless, I could see where some high temperature gasket could potentially help prevent smoke from sneaking out causing staining. I would just caution that some PG500s run very close to the edge in terms of pressure buildup inside them... and if you start plugging every nook and cranny where smoke ( and/or pressure ) escapes, the only place left for it to go in addition to the exhaust chimney , is out the auger tube and then out through the hopper. Don't forget about all the places near the ash tray and warming drawer, there is potential for smoke leaking out around those areas too.