Building a mosaic should be a transparent operation

Some users at FOSS4G lamented the fact that mosaic building offline is cumbersome and error prone. Some commercial imagery software allow one to just point the software to a directory and have the mosaic built for them transparently.
This is an interesting concept, and should be doable, the only issue I'm seeing atm is the time needed to build the mosaic... it should not take that much, but what if the number of files is really huge or the metadata to be read "rich" and the http request times out in the meantime?
Anyways, with the new UI we could have a custom UI for mosaics that uses ajax to deal with the long operation and report back progress.