I would not be able to say whether this would be something in the next build or not. There never is a definative answer on what will or will not be included in new builds on most software. The cycle works like this here and for most companies.
Basically there is no active beta on Photopost currently but once a beta opens the developer looks at our suggestions forum for ideas of what customers want by suggestions they give. He may also have some ideas of his own. He then prioritizes the list and figure out what is feasible and in what order things fall and begins working on the code. In every cycle some things get added some things do not.
As far as what your looking to do you could easily modify the code to do this with some code additions and a new sort. Basically as simple as adding a new mysql field to the photos table and a query in comments.php file when a comment is added to up the count in the new mysql field. You can then add a sort to the sort table to order by that column.
Our reviewpost product does not have a specific sort by max review sort but it has the max review field in the table so you can sort by it if you add a new sort to the sort column example here. See how it shows how many reviews each product has.