I see your point, but I think I may still be able to figure something out. My foregrip-mounted tac package would only be about 600g (eschewing the use of a ~220g side-mounting RIS). Also, at least for airsoft purposes, aren't the lengths of the handguard top half the same for all AK series?
In this photo, both AKS-74 and AKS-74U seem to have identical lengths.
The Dboys system is attached in the photo so you can see it. The handguard is mounted/removed off the handguards by twisting it on the horizontal axis, and aligning the ridge on the handguard with the channel in the tube's end brackets. So it got me thinking, I could take my chances and run to Huang's AirsoftDepot and
check out if I can mount the top half of this rail onto the battery-tube using the same technique. If I can, I think I found my solution.
Now that would make any wobble unacceptable, since I'll have optics mounted on there. But I believe that is simple to solve with some Gorilla glue and improvised spacers.