I'm considering moving my mother's washer and dryer upstairs where there's oak flooring. A leak would be really bad news so I'm probably going to put in a pan underneath the washer. I'm assuming this also must go to a trap, but since it won't (hopefully) get used much, if at all, what's the best way to keep that trap primed? As an aside, would it be legal to plumb it outside directly? That would be one less trap and vent to deal with and a one-time discharge outside shouldn't be a big issue.

This is an old house (built in the early 50's) that has inadequate venting per today's standards. I could probably get to the one vent upstairs but it would likely mean tearing out plaster for two stories to get to above the flood plane of the things above. Would an AAV (I'm pretty sure they're approved where she lives) be a viable alternative? The inspector did allow me to use one when I put in a new shower for her a few years ago. If it makes any difference, she's on a septic system.
Not sure how effective they are on a WM.

I like the look of the Sioux Chief Ott boxes for washing machine supply/drain with the suppressors integrated. Who makes a decent washer pan? I've not looked around for one.