I managed to install Lede 17.01.4 on my UniFi AP AC PRO. In principle everything works fine. Now, I would like to install some VPN Server packages but unfortunately the /overlay partition is rather small:
You can combine the "ubnt-airos" and "firmware" partitions into one big "firmware" partition, assuming you don't want to keep the airos as a backup. OpenWrt does not support the dual firmware partition on ths device anyway, so there is really no point in this split unless you use it to toggle between OpenWrt and Airos without a reinstall. Not that there would be much point in that either...
You should probably verify that you are booting from the "firmware" partition before making the change. If you flashed OpenWrt twice from Airos, like most installation guides tell you too, then you might actually load the kernel from the "ubnt-airos" partition. This will not work well with combined partitions.
The "bs" partition contains the flag used by the bootloader to select which of the two firmware partitions to boot. It should look like this when from "firmware" is the preferred:
It's simply single bit flag. Writing a utility (or as part of the mtd tool) is easy. But the main issue is to enable OpenWrt to actuall run from the second partition. The bootloader will of course happily load the OpenWrt kernel from the second, but the rootfs partition will still be split out from the first. You'd need some extra driver magic around the mtd partition splitting to run the rootfs split on the second instead. Probably not much point, but I wonder what the benefit is? Might be better to just say that the OpenWrt partition scheme is different from the OEM firmware, and have twice as much space.