It's difficult. But as far as I understand it (and I had german contract law for two years) the OGL says that every variant rule you make for an open content rule, is also open content automaticaly. For example, the Combat Maneuver Roll system from the Alpha pdf would be completely open content and everyone can copy and modify it as desired. As Paizo didn't owe the rules for disarm and trip, they also don't owe the variant rules.
But on the other hand, there's copyright. You are not allowed to copy any text even from a OGL publication.
The tough thing is probably really original rules. For example take Sudden Strike. It's not part of the SRD, but in some way it's a sneak attack variant. You're clearly not allowed to make a sneak attack variant and call it sudden strike. But creating a variant rule of sneak attack that works exactly the same way and you describe it without copying the text from the WotC book?
That's the thing that is not completely clear. And when you get really new stuff, it becomes even more difficult. Take the beguiler class. Is it generic enough that Wizards can't claim it an original creation protected by copyright? That's something that really isn't easy to say.