Nice work guys!
Unfortunately, according to bbraun's testing, it's looking like 040 Macs won't support addressing ROM bigger than 1 MB in size -- at least in the Quadra 650 or the Quadra 700. It appears to be a limitation in the actual ASICs on the logic board :-( That pretty much eliminates the possibility of a ROM disk on these models. You could still use a custom ROM to change the startup chime though...
Unfortunately, according to bbraun's testing, it's looking like 040 Macs won't support addressing ROM bigger than 1 MB in size -- at least in the Quadra 650 or the Quadra 700. It appears to be a limitation in the actual ASICs on the logic board :-( That pretty much eliminates the possibility of a ROM disk on these models. You could still use a custom ROM to change the startup chime though...