Did some experimenting and found that the GHK G5 magazine internals can fit into the GHK STANAG magazine internals and vice versa.

Here's the magazines how they arrived. These two in particular were reported leaky by the seller.

Removing baseplates from both mags. The STANAG baseplate end needs to be lifted and slid off, while the polymer G5 mag needs a lot more effort to sliding it off. Be careful too as the BB spring and follower assembly will probably shoot out of it if the bottom plate comes off entirely.

Removed 'guts' of magazine from their respective shells. The STANAG mag had a (missing) screw at the bottom to hold it in, the G5 mag has a pin hidden by the baseplate when it is installed.

Completely disassembled. Note how the two gas chambers are near identical, save for the STANAG mag one having a part extend out below the gas route in order to capture the screw holding the BB follower assembly together.

Holding the STANAG BB follower assembly up to the G5 mag gas chamber, it mates up properly with the curves and alignment braces even though it's missing that part that the screw goes through to secure it.

They both fit back into the other's outer shell. The floating BB follower assembly plus G5 gas chamber assembly in the STANAG shell, and the STANAG gas chamber with its extra wing for the follower assembly in the G5 shell with its independent BB follower compartment.

I don't know if that floating BB follower assembly will cause feed or spacing issues under more strenuous usage, but for testing purposes both mags cycled the gun and fed properly. The pin for the G5 mag did not quite line up with the hole in the STANAG gas chamber, however it was by such a slight margin I am assuming it was due to wear and tear on the mags over their lifetime. I believe the pin for the G5 mag was also bent in the original gas chamber so the hole might have deformed as well. The bolt catch worked properly, although sometimes it did catch open one BB too early but that may be due to the fact that the bolt catch lever is slightly bent downwards.
Either way, the G5 shot successfully on full auto with both of those swapped guts mags. Things are looking good to convert all my mags over to the newest GMAG standard from GHK.