The OP's solution seems overly convoluted. Why not just add a token system, instead? Each run gives you a token. Exchanging a certain number of tokens will allow you to obtain a piece of gear of your choice. You may only purchase one piece of gear per week, and doing so uses up your one drop per week limit (similarly, getting a drop from a run disables your ability to purchase for the week). By putting the token at the end of the run, folks will be incentivized to stay for the whole run, even if they get a drop (they can apply the token toward another piece of gear next week). At the same time, it puts a limit on the number of times you need to run to ensure you get a piece of gear.

The worst I can see from this is that folks might stockpile tokens; run the raid sixty times in one week, for instance, and then just buy a piece of gear each week for the next several without setting foot in the raid again. I don't think this would be common behavior, however.