Cute idea.
Be aware though that a primer firing generates a remarkable amount of thrust, pushing the case forward very hard, before the powder ignites. So I'm not sure if the o-ring would actually hold the case aft firmly enough to accomplish the desired goal. You could easily test this though - take a primed empty case, chamber it (with o-ring installed) and fire the primer. Extract it, and see how far the primer has backed itself out of the case head - that's the amount the the case was pushed forward during the primer's firing.
If you have a cartridge case headspace measuring gauge for a rifle that uses a rimless case (e.g. .308 Win), measure a case's headspace, prime it and fire it (primer-only), then measure the case's headspace again. The primer has enough energy to push the shoulder back a few thou.