Radio Shack used to have a cool little thermometer which had indoor/outdoor temp readings. The outdoor temp was taken from a little probe which was on the end of a length of small wire.
I just checked, and they seem to have gone to a wireless model for a bit more money: $39.97 for a Model: 63-1089
Catalog #: 63-1089
That would be the easiest way to get a realtime readout of the spa temp.
As to modifying the panel, I don't know of a way to make it read the current water temp, but if the green light is on, it is usually within a degree of the set temp.