DV-
The best solution I can suggest to you is button snaps.
My seamstress turned me on to them when my costumes were made for C4.
When properly stitched in, the buttons will resist virtually all slippage issues; there are very few select instances that will cause a snap to dislodge.
Also consider this observation: with opposing buttons on the shoulder seams (i.e. the 'innie' on one shoulder, the 'outie' on the other... sorry, I couldn't think of the appropriate terms...), it's guaranteed that the correct tabbard will be on the correct shoulder.
PM/email me for any specific questions.
Tim the PokerJedi