I was looking into this the other day. The cradles look similar, but according to the GPS City "Community Forum" the answer is no - they cradles are slightly different.
Here's the post I saw.
That said, it does look like the 590 cradle would attach to the Can Am mounting bracket. In other words, the four mounting holes would likely line up. So you'd have to remove some tupperware and wire in the new cradle. I don't know if any modification of the harness would be necessary, but if so, I would think that it could be done easily enough.
However the Zumo 590 is a 5" screen and the 660 screen is only 4.3". The questions is... Will the larger Zumo 590 fit between the RT handlebars? I don't know the answer, as I did not look at the overall dimensions.
Sure would be nice if it all worked. The user interface on the 590 is a generation apart from the older 660 and is wonderful to work with. I have the new interface on the RV 760LMT that I use in my motorhome, and I love it.