In other words, "Here's an extra $5k for making the playoffs" is flat-out illegal.
What is 100% legal, and what happens frequently, is "Here's a $5000 bump in your stipend because of all the hard work you're doing, Coach. Thanks for your dedication to our district/campus/kids." Equally legal is "And we're going to make it retroactive to the start of this school year due to a renegotiated pay scale the Board approved" even if it's skirting the line.
If you're in a small to medium town, or a "lesser" program, you'd better believe that these bumps happen on the regular to retain successful coaches. That's why many districts don't even post their head FC stiped on their websites - it's renegotiated annually.