Without getting into the details of why I want to try this, is there any issues with using RO water for a new fill that I need to be aware of other than adding calcium?

Not really sure what you mean by "RO water". Typically, reverse osmosis removes dissolved substances that are in the water and then the water, now free of these materials, is returned to the pool that it came from. So the RO water virtually has far lower dissolved solids than before and this is a good thing.

I agree...adjust CH, TA and PH to withing recommended levels and you are good to go.

If you are planning to use a household RO system, you need to be aware that they only process a few gallons a day.

Thanks for all the info. Just wanted to make sure that normal balancing of the water was all that I needed to do. The water I normally use is pretty hard (350 CH). I have a source of bulk RO water that I was going to have hauled in.

Can you get a read on what you are getting back with the R/O water? If your spa is fibreglas, then your CH levels would not need to be the same levels as needed for plaster, although you will obviously still need some.

Other than that, I can't see any reason why anyone would want anything other than R/O water in their pool/spa