I'm not sure there is really a good answer to this problem without custom hardware. General wireless products aren't really going to be a great option as wifi and bluetooth will both have variable latency issues and missed packets. What you'd really want is something akin to flash triggers, but capable of firing repeatedly at your frame rate. It would need to be a custom wireless solution for the problem though.
On the software side, you can try frame interpolation, but I'm not sure that the quality loss associated wouldn't be worse than the minor timing difference of not being gen-locked. It still isn't going to match up exactly as it is a guess rather than what actually happened.
This is also ignoring the fact that rolling shutter greatly reduces the advantage of gen-lock to begin with. The sensor itself is taking time to read the scene and so not even the top and bottom of the frame are truly in sync. Genlocking is really something that more mattered for doing live switching where you needed to be able to keep a constant frame sequence and needed a frame to send. It can also be useful in fine tuning audio/video sync, but that's pretty secondary to the technical needs to it, especially if you are shooting high framerate video.
That may actually be the real solution to your problem too. If you shoot at a higher frame rate than your intended final product, you'll have frames much closer in timing to each other and can pick which frames to use. This will work best if it's an even multiple, but pull downs may also work acceptably. So if you shot 60 fps and were planning on going with 30, you could pick your frames to pull out to make sure you were at most half a frame off and on average be only 1/4 frame off. That's a pretty tight sync without needing any weird magic.
