That PRAM battery is going to be scourge you do not want it to be. I'm hoping that the PRAM Battery is not hidden in the case of the 500-series like it is on the 5300 and G3 Wallstreets - when you separate the case and remove the connected wires, in the 5300 and G3 Wallstreets there's a red & white wire that goes into a folded piece of plastic on the top case - that is where the PRAM Battery is. Its a 2032 or similar watch battery with the wires solder-tabbed onto it.

In short how about replacing the PRAM Battery on the 540c, and if need be, use a watch battery like many PCs do. Since the 5300 and G3 Wallstreets use a 2032 watch battery, why not use one of those. Make it easy, get a watch battery holder and solder to the board or someplace and replace the battery as needed by sliding into the battery holder. I think somebody put a post of how they did that with their Mac II Series, and it's a great idea - watch batteries last for a couple of years and are relatively cheap.