Your question intrigued me, even though I haven’t dehydrated anything in years (some things tend to dehydrate naturally here in the desert. People included, lol). So I did a little research. Most recipes I found didn’t specifically ban oil, but none of them included it. But it appears you can use oil when you dehydrate mushrooms, although it greatly shortens their shelf life. You can extend their shelf life by vacuum packing them.
If you want to season the mushrooms, one site suggests adding your spices to water, lemon juice, or vinegar, then brushing the mushrooms with it. The lemon juice or the vinegar would be better than water, I imagine, because they would add some flavor and tang, but I don’t know if that is desirable in mushroom bacon.
I’d never heard of mushroom bacon until I read your post. I’m no vegan, but I never really liked American bacon. Mushroom bacon sounds like a tasty and versatile replacement (not to mention much healthier). It sounds delicious! I’ll have to either find my old, noisy dehydrator or run up my electric bill by keeping the oven on for twelve hours or so, but I want to try it! Thanks for the inspiration.