As stated in this answer:
We're working on adding a "hot" sort to the filters feature. (Currently, the only sorts are "activity", "newest", and "no answers".) When it's finished, you'll be able to create a filter with all questions from a specific subset of sites, and then select the "hot" sort.
Date June 2011. Are there still plans to implement this, or did it fall by the wayside?
I love browsing the "hot" questions on Stack Exchange but there are plenty of sites I simply don't care about at all (e.g. Latex, Christianity, RPG). It would be really great to be able to pick only the sites I care about.