I think it will work as an RGB monitor for a test rig. The following list was from an apple forum regarding the GS monitor and others that are compatable including the Commodore that you mentioned.
You should keep an eye out for any analog RGB monitor that can horizontally sync down to 15.75 kHz. These include:
AppleColor RGB Monitor replacement (model A2M6014, the IIgs monitor)
Commodore 1084 models and equivalents like the Magnavox Professional 80
Sony CPD 1302 MultiSynch
NEC MultiSync (original), MultiSync II, MultiSync 3D
Commodore 1902 (although most of these only do digital RGB)
Tandy CM-8 (not sure about model but it was used with the Coco3)
Atari SC1224 (used with the ST series)