I don't believe that this can be done directly.
One option would be to use the rebase tool to move the appropriate parts of the repository to the other branch after the pull. using something like the following steps:
- Create desired branch, e.g.
hg branch release-1 - Commit new branch:
hg commit -m "Created branch" - Pull the remote repository to the default mercurial branch:
hg pull perforce-R1 - Rebase the new pull to the desired branch:
hg rebase -s <first changeset of perforce-R1> -d release-1