... but it's not up to the listing agency to pay that fee.
You can do whatever you want, but keep in mind that if you are a member of the mls, you must agree to pay the cobroke amount. There's a difference between a cobroke fee and a buyer agency fee.
I think you're business model is flawed to the max.
If you want to offer variable cobroke to different brokers, no problem. Do it ahead of time so we can filter the IDX and display only listings that will pay a fair cobroke. I'm sure we can get a tech to get this done.