Out of curiosity, has anyone with one of these phones tried doing this?I think if you use the GrapheneOS installer tool you mentioned here to unlock the bootloader, you should be able to do it. I would suggest trying to use the GrapheneOS tool to unlock the bootloader (don't do anything else with it though). After that, I would suggest live booting TWRP using `fastboot boot YourTwrpImage.img` (don't flash it to the phone's recovery partition since you won't be able to get the ArcaneOS recovery back if you do!).
After that, try using the instructions here under the "This is How to Make Complete / Full BACKUP with TWRP" to take a backup of the phone. I believe those instructions would also include the recovery image in the backup, but it would still be a good idea to explicitly take one in case I am wrong, the instructions for explicitly taking a backup of the recovery are available in the terminal commands here.
After doing all that, reboot into ArcaneOS and copy the images you made off the SD card to your computer! Anyone who wants to then try ArcaneOS on their devices should be able to write the images to their device as long as it is the same phone and give ArcaneOS a try for themselves!
Needless to say, without having an ArcaneOS to try this on, I cannot promise that any of this would actually work, but it's worth a shot! One other thing I should mention here is that using this method to save off a copy of the image would retain any encryption on the device if any was used, so you would want to make sure that any passcodes you use on the device would be something that you are comfortable with sharing.
Additionally, it would be a good idea to share the commands that you used for taking the backup images. Output from the `dd` utility which is used for this can be passed through gzip for example to compress it further, but as a result, anyone who wants to use the image would need to know that so they can decompress it.