We have plans to support comet and to provide an extensible messaging system in the backend that might be even extended to support the the google wave backend. This is some thing that obviusly was not available when we did our blue prints for the comet support and we wil have to look into in terms of interopability with XMPP.
Regarding scalability I guess that the following the best practices of comet which basically entail a separate domain for the comet calls and still maintainig the ability to interact with the primary domain is the secret souce for creating the solution scalable. Because of our focus in 6.4 I don't think we will be able to squeeze in any actual work on the subject, but I guarantee that in 7.0 we will look into providing a transparent way to implement realtime messaging and updating with in Visual WebGui. I also suspect and based on the interaction we had that migrating from the solution you guys are doing to the official API will be simple enough.
I believe that your effort to make it work on the existing version will not be in vain as if we are targeting the 7.0 version that means that it will take some time to mature. I guess that the feedbacks you guys will have from you attempts will help to create a better solution when we start implementing the official solution.
Keep on the good job.. :-)