I have been able to backup my recovery partition with ASUS Backtracker on versions 3.0.7, 3.0.8 and the newly released 3.0.9. I used a SanDisk Extreme 16GB USB 3.0 drive, model # SDCZ80-016G-GAM46.
Whether or not the backup actually works is another story. Since I'm not really worried about losing any data on my 1TB drive I'll find some time to remove my M.2 SSD and use the HDD as a testbed for the recovery, then report back.
EDIT: Sorry, forgot to disclaim I own a G751JY in case that wasn't observed in my displayed PC specs.
UPDATE: Factory restore using the ASUS Backtracker image on the aforementioned USB drive was successful. My laptop booted as you would expect with an out-of-the-box setup. The recovery partition is also back on the storage drive, but I noticed this time it's 20 GB instead of 12 GB as it was on my M.2 SSD. I can't say with confidence I know the reason, but my guess would be, similar to ASUS Backtracker, the recovery partition scales in size depending on the capacity of the storage. Just under 4 GB are available on my 16 GB USB drive after using Backtracker, but when I used a 32 GB USB drive the recovery used 20 GB of space.
I'm not going to tell you that you'll have the same fortunate experience as I just had given the mixed results people have shared on this forum. However, I'm guessing I'm the first guinea pig among G751 owners to give this a shot with the latest version on a USB stick reported to work and share working results. If possible, I'd recommend trying to grab the latest model of Sandisk's Extreme USB 3.0 drive if you don't already have one.
On a somewhat related note, it's worth mentioning that due to Microsoft's stupid certification requirements for USB sticks to be Windows 8 certified, SanDisk ended up configuring an unknown number of USB sticks as "fixed" drives before switching back to "removable". I don't know if this makes a difference in the recovery process, but I made sure mine was configured as "removable" before proceeding with this test to be on the safe side.
I wish you the best of luck on securing your recovery partition. If you get to the point of creating a successful backup on a USB drive, test out the recovery on a dummy drive when you can. Of course, please do share those results with the rest of us. 🙂