Most clocks chips I've looked at, I've not looked at the Amiga, tend to work down to 2.2V or so, so its probably better to use a schottkey diode as they have a .3V drop, which gives you a bit more wiggle room on a 3V lithium battery. But a standard diode should be fine, it will just have a shorter battery life.
I think it holds true for Amiga gear,too - aamof my A500 memory board had just a 2.4V NiCad battery.
