After the past 4 hours of fighting with this, I figured it was worth me posting.
After a lot of messing around, I finally searched the OpenQA forum (which is apparently not adequately indexed by Google, since I did search on Google a number of times). Specifically, I found a forum thread, linking the issue to Issue 558. Specifically, a change in the way that Selenium treats windows means that only ’safe’ properties could be touched — things like .body, .title, etc. — which explains why I could test window.location, but not window.map.
In any case, while investigating this, I built a new version of Selenium IDE, which adds the getUserWindow function, and repackaged it. This is a change directly from the 1.0b2 XPI I installed, and implements the fix described in the forum thread linked above.
This applies especially to functions like assertEval / verifyEval / getEval and its partners.
In order to take advantage of this, you must change any instances of ‘window’ or ‘this.browserbot.getCurrentWindow()’ to ‘this.browserbot.getUserWindow()’ where they need to access user-set properties. This simply acts as a transition tool for people needing 1.0b2 support and unable to wait for another release for this function to become available.