1. Flashing P10 firmware to the Mate 9
Surprisingly, it is possible to flash P10 firmware to the Mate 9, and it boots. A Mate 9 can even be rebranded as a P10. Unfortunately, there is something different about the Wifi driver, and Wifi does not work. It seems the mobile radios, bluetooth, etc. do work, though. I wasn't able to test this very thoroughly, and I don't recommend anyone else does, as it's impossible to recover it even with offline firmware, but it would be possible in the future for someone to build a ROM based on P10 firmware, I'm pretty sure.
2. Android O
Huawei has an internal Android O build for the Mate 9. This build comes pre-rooted for Huawei's internal development use, which opens up the option of rebranding without ever unlocking the bootloader, and doing other such tricks. Apart from looking at it and seeing the Android O features, the build is pretty useless as a daily driver. Things like the play store can't be installed easily. The recovery which comes with the build has broken Wifi, and I needed to use a HiSuite exploit to get it to restore to previous versions.
With both of the above mentioned firmwares, the recovery system is different. The TWRP which we currently have for the Mate 9 does not work. Rebuilding the recovery image with the tools available also does not work. Something's different about the way the recovery image is made, but I can't figure out what it is. So it's not possible to have TWRP yet in either of these environments, which makes them much less useful.
Even more interestingly, the P10 recovery images which do not work when flashed to a Mate 9 running EMUI 7.0, do work if the Mate 9 is running Android O build, or if it's running the P10 roms.
It's very easy to tell which environment it's in, as the EMUI 5.0 Recovery has a light blue theme, while the Android O / P10 builds, and so on have a dark blue theme.
It should also be possible to flash Mate 9 ROMs on the P10, which would let P10 users access TWRP (but they would lose Wifi in eRecovery, making it very, very hard to revert to P10 ROM)
--What from here?--
Eventually, these updates will be released for the Mate 9, and we'll need to rebuild TWRP. A nice side effect is, if we build a working TWRP for the P10, it will also work with future Mate 9 builds. I'm hoping that the next internal EMUI 5.1/Android O build will have Wifi working in eRecovery to make it possible to do more research on this with an easier recovery method.
For the sake of research, I will upload and link to the recovery img from the Android O build. Feel free to poke around with it! Note that just flashing this to your Mate 9 running EMUI 5.0, it will not work. Hopefully TWRP based off of this can be made. I've tried and have not been successful building something which will boot, despite it being easy for EMUI 5.0-based recovery.
Link for Boot, Recovery, Recovery2 from EMUI 5.1 / Android O build for MHA-AL00: