The answers above were most useful and I learned a lot. However, for my needs the succinct answer is:
hg revert --all --rev ${1}
hg commit -m "Restoring branch ${1} as default"
where ${1}
is the number of the revision or the name of the branch. These two lines are actually part of a bash script, but they work fine on their own if you want to do it manually.
This is useful if you need to add a hot fix to a release branch, but need to build from default (until we get our CI tools right and able to build from branches and later do away with release branches as well).