So I have an idea for a hack. I have the basic mechanics (mechanic changes) down in my head.
How do I go about playtesting this stuff as best as possible? Any playtesting advice from BWHQ?
Write it down, find a group, and play! 
Here’s some things to keep in mind.
Try it out yourself. Do what you can to give the mechanics a quick spin on your own. Sometimes, this reveals really glaring issues that you didn’t notice when you put it together in your head.
Take notes while playtesting. Make observations about the expected and actual behavior in the game.
Fish for feedback. Have specific, targeted questions to ask players afterwards. Prompt them for particular thoughts instead of asking “was it good/bad?” Know what you want to ask about. Are there any concerns you’re aware of?
Don’t alter the hack in playtesting. Wait until the session is finished. You need that valuable feedback!
Try not to go on the defensive, also. It’s tempting to protest when they offer feedback: “But I put X in for a good reason!” or “Y isn’t a problem if you’d just do this one thing you’re supposed to do!” Instead, try to figure out why they didn’t like X, or why they didn’t do that one thing. Likewise, if you’re testing the rules document as well as just the mechanics, don’t explain or clarify or suggest anything. Just see what they do with what you’ve given them.
Oh, and that reminds me!
Player feedback is a symptom, not a diagnosis. When players offer suggestions, they’re offering ways to solve the problem that they’re unconsciously noticing. Try to find the common elements between the feedback. Daniel Solis explains it in detail here.