Whatever makes you feel good
Eric, this question has been around a long time, and doesn't need to be personally justified to the crowd by the individual pilot IMO.
Yes, you could fit a seat pack in your airplane, although you would need to take the bulk of the container into account when designing your cushion system. A slim pack container might even be a better option. No problem there.
Regarding the tip up. I don't see how you could ever get the canopy open enough in controlled flight to exit properly. Once the canopy opens, the center of lift changes and many people have reported significant nose pitch. If you could get someone to stay behind while you jump, then may be that would work.
If you could somehow successfully jettison the canopy, there's no proof or demonstration that it would clear the aircraft during the event. or how the aircraft would handle with the canopy off. And then there's the issue of you clearing the tail on the way out, although done correctly this could managed.
Another side concern would be that having that 'chute would lead you to believe that it would save you in the case of a bad situation.
BRS has a whole other set of issues, but since you can't get it on a -12, there ya go.
I've jumped out of aircraft hundreds of times in a controlled skydiving environment, and I personally wouldn't consider using a parachute in a tip up RV as an option.
All my personal opinion, hope it helps...
Cheers