Calculating XY coordinates in Decimal Degrees from projected dataset in model or script?

I think you can.

Just define the Spatial Reference in WGS-84 and use cursor using this Spatial Reference.

Coordinates are specified in the spatial_reference provided, and converted on the fly to the coordinate system of the dataset.

Fore more detail see this.

I'm not sure about a script, but I've this accomplished in a somewhat automated fashion using model builder: import your xy coordinates and project them into WGS 1984. Then add fields and calculate the geometry of the points in decimal degrees. Then bring your XY points back into the original coordinate system.