It appears to me that the 8 speed Ergo's should work with the 7 speed RD. The 7 speed freewheels and the 8 speed cassettes have the same cog spacing (5 mm), and as long as you don't tell the RD that the cable is now attached to an Ergo mechanism at the other end, I think the same shift should result. Only thing, maybe the 7 speed RD can not make the full movement, but I think it will.
I used 8 speed Ergo's with 7 speed freewheels for quite some time, it worked flawlessly, but there the RD was matching.