Rivatuner also can adjust (????) voltage.
/wi1,71,18,3c - Default 1.20v setting.
It is easier with GPUTool though. I'll give you that. :)
But they don't.
Newer HD4890's built with non-reference PCBs (Sapphire Vapor-X, MSI Cyclone, XFX ZDFC) lack the necessary Volterra VRM chip(s)...