You must mean the TRX500FA/FGA Rubicon .. The TRX500FE/FM/TE/TM Foreman doesn't use a high pressure oil pump .. And the TRX400FA/FGA oil pump is gear drive..
Yes the TRX500 Rubicon oil pump chain will/can come off once it's worn/stretched if you bump into something with the bike , not all will do it but some will , we've fixed a number of them..
What are you mounting the tensioner on..?.. About the only 2 mounting points there are the 2 posts that hold the oil separation plate , surely your not leaving the plate out..? .. If your repairing a lot of these bikes then you will be able to post pictures of the tensioner ..