FWIW, I recently bought a second Unifi UAP AC Pro. It came with firmware v3.7.58. Was prepared to downgrade it to be able to install OpenWrt, but tried a direct install just in case. And to my surprise that worked just fine! This is all I had to do:
BZ.v3.7.58# mtd -r -e kernel0 write /tmp/openwrt-18.06-snapshot-ar71xx-generic-ubnt-unifiac-pro-squashfs-sysupgrade.bin kernel0
Unlocking kernel0 ...
Erasing kernel0 ...
Writing from /tmp/openwrt-18.06-snapshot-ar71xx-generic-ubnt-unifiac-pro-squashfs-sysupgrade.bin to kernel0 ...
Rebooting ...
Connection to 192.168.1.20 closed by remote host.
Connection to 192.168.1.20 closed.
So I guess they must have removed the bootloader restriction in these newer firmwares for the UAP AC product line?
I am almost sure that the previous UAP AC Pro I bought (with v3.4.14 firmware) refused to boot OpenWrt before I downgraded it to v3.4.7. I say "almost" because I've become uncertain whether that was just a confusion over the dual image feature. The install guides were (are?) quite misleading, giving the impression that you could install OpenWrt in either the kernel0 or kernel1 partitions, when OpenWrt in fact will only be able to run from kernel0. This might give the false impression that the bootloader refuses to boot OpenWrt. Especially since (almost) no one connects a console, given that the box is glued and there is no factory installed header.