Again I am no lawyer, but you have two courses available to you. You use the Year Zero Engine OGL, without mentioning Forbidden Lands compatibility, and without using any of the Trade Dress of FL, or, you approach Free League and negotiate a licence agreement.
I find there's lots of uncertainty about this sort of thing. Companies can create licensing opportunities to let people do things they can't already do, but creating a licensing program doesn't mean that all other previously legal avenues suddenly close.
For example, if WOTC created a "Reviewing Wizard" program, it would still be legal to review WOTC products outside of that program, it's not in WOTC's power to grant or withhold that ability. What you can't do is call yourself a Reviewing Wizard™. The hypothetical Reviewing Wizard license grants people new rights (e.g. use of the trademark Reviewing Wizard™ in a manner that implies a relationship or endorsement by WOTC), in exchange for new restrictions (perhaps wizards can only write positive reviews). Both sides get something they didn't have before.
Compatible products are generally legal—consider the third-party razor blades for Gillette razors, off-brand inkjet cartridges, iPhone-sized phone cases. In fact, it's even legal to refer to trademarks to explain this compatibility
, as long as you're using the minimum reference and not creating impressions of a business relationship or endorsement (e.g. by using their logo).
So it's fine to produce Pathfinder-compatible products, and to say so; what the Pathfinder compatibility license
grants you is new rights: the use of their trademarked "Pathfinder compatible" logo, and a font. If you don't use that license, you don't get those things, but you're still able to do anything you could have done before they created that license.
If you're not using trademarks in a manner that implies a relationship or endorsement, you're not using any copyrighted materials or (I guess) patents, what are you licensing?
That's my understanding, at any rate, so that's what I'm here to check out.