When I used to ride and race dirt bikes, we used to put that info on a sticker on the back of the helmet. Guess you could do the same with your mountain bike helmet. We just used those address labels that you can run through your printer. I also wear a Road ID when I ride alone with at least my info and my wife's contact info in case someone finds me wadded up on the side of the trail somewhere.
I keep a copy of my DL (with medical ins card info on the back of it) in the outside pocket of my pack, I also put the personal info/emergency contact sticker on the visor of my Bell 2R and I use Road ID and have my medical info on my phone for the "medical ID" function on the lock screen. I had an ICEdot profile that came with my helmet but didn't renew it since I think I am covered with all the above.
The sticker idea made me think of getting some stickers to print and apply to medication bottles since the one I'm caring for can't read the tiny type on the pharmacy label.
I just got swarmed by bees while pedaling home from work. 7 or so stings on scalp (two went under my helmet!) and back of neck. Made quite a scene in the middle of the street tearing my helmet off and windmilling my arms around. Good thing im not allergic!
(well, everyones a little allergic...)