I agree with Marti, it wouldn't be bribery - unsatisfied customer would not really interested in a coupon code, but it would be encouraging the satisfied customers.
Here are two facts about buyers:
1. They forget! So just convo or e-mail a little reminder to your customers by mentioning how vital the feedback is for your business on Etsy, they will probably leave one ASAP.
2. For some weird reason, discounts make people happy! You can require your "bargaining" customers to leave a feedback within 30 days after they receive their item(s), otherwise you will charge the amount at the end of 30 days - as a part of the deal.
Also, don't forget to provide them a list of places they need to click to get to the feedback section ("Purchases & Reviews" under "Your Account" section at the top right of the page). IT'S REAL COMPLICATION AFTER THE LAST UPDATE!!!
These should work effectively, and if they're not still leaving any I would not really push any further (I guess the reason is obvious).