Here's a link that may help you out a little bit I found a few years back.
http://mikereidconstruction.com/Washington%20Horse%20Trails/downloadable_maps.htmLook on the west side NF websites for horse camps too. A couple that come to mind off the top of my head are Kalama Horse Camp and Mt Adams Horse Camp. Just google both the camp names.
I spend most of my riding time on the east slopes of the Cascades, simply because I like it over there. Dry camping, highlined, day rides out out of camp stuff. Once in awhile an overnighter. Many of the places I go you could get your MH too.
PM me if you want some east slope ideas.
And no, I don't take my horses hunting here in WA. I modern and too many wingnuts. Not to mention my new #1 trail horse is a buckskin and I don't think I could hang enough orange off him to keep some yahoo from thinking he's an elk.

Just remember, NF requires certified weed fee hay and hay products if you choose to stay on NF land.