Any know if it is possible to disconnect the fan and just use the Tower and as a big Hard Drive enclosure???
You can't do that while the drives are connected to the logic board. Though the requirements are minimal. the processors are still fuctioning, thus the fan activity.
Disconnect the fans and you will have a box full of fried components.
Add to that the continued leakage of the LCS ands you also have a fire hazard.
Gut the G5, scavange some hard drive enclosure control boards and power supplies (or one ATX PSU), fabricate the needed racks and connectors, and, Yes, the tower could be a large hard drive case.
A G5 consumes upwards of 150watts just at dead idle, compared to a FW tower enclosure at 50 with the drives.