If you used a 660AV logic, you could use a color TV CRT as your internal monitor, hooked up to the AV port. Then use the main logicboard video out for an external monitor. You can force the AV Quadras to use the composite out as the boot screen, IIRC.

An 840AV logic would be too large. Another alternative might be a 6100 with the AV card. G3 optional