Any object that is serializable should be transferrable over the wire
to the server. I just looked and it doesn't seem like
JavaScriptObject is serializable, so you are probably out of luck.
I'm reasonably sure there are (maybe third-party) GWT libraries for
using JSON communication.
As for your second point, I would assume this to work on the server,
but I haven't tried it. The only reason it wouldn't is if it used
JSNI, which is certainly possible ... you'd need to check the source
code to verify.
Hope this helps.