Here is an idea. You loose a player you get the teams pick that the player was drafted at of the team that signs him. If he is an UDRFA It's 2 7th rnd picks at the end of the draft. If he is a FA signing (Stallworth) you get nothing.
That's essentially a variant on restricted free agency which would pretty much eliminate unrestricted free agency.
Wouldn't it just be easier to lay out the parameters of the "formula" and make it public?