One thing to make clear: simply using #! does not make your app crawlable. Your server-side app needs to also handle URLs that contain _escaped_fragment_, per Google's specification:
Would it be possible to use pushState as the main mechanism on the client, and use a #! as a fallback? You would end up with two different possible URLs, but if the same library handles the calls then it would be fairly straightforward to move from one to the other.
If you do capability detection by checking to see if window.history.pushState exists, then you can configure RouteMap.handler to be tied to history events. Otherwise, you can set the prefix by calling RouteMap.prefix('!').
EDIT: I re-read your question, and I think that I misunderstood. I do think what you're asking is possible, and what I stated above is a piece of what you'd need to do. Basically, instead of simply binding RouteMap.handler to either onhashchange or onpopstate, you'd probably have to write an event listener for both of those methods + a window onload handler all of which send their respective portions of the URL that you care about to RouteMap.handler.
Not to be overly pedantic, but a piece of core functionality like this seems like it would really need some basic automated tests, even if they were just running the API through it's paces.
Thanks for the suggestion. We haven't actually decided on the larger project's testing scheme, but once we do, we'll definitely be adding tests to RouteMap as well.
http://code.google.com/web/ajaxcrawling/docs/getting-started...