I'd use x,y coords since they'll likely be in different positions. Chances are they'll only need top-left, bottom-right, center, etc...
Here is something I have used before... Allows you to store Client side variables without cookies... http://www.thomasfrank.se/sessionvars.html Could be handy. fyi, these are not secure variables, so I wouldn't use for sensitive data, but for dialog IDs it should work pretty good.
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