If the changes done to <branchname> will be merged with default (to fix a
problem only in the release and merge it into the devel/default branch), you
will have to do a merge in the devel repo anyway, so the warning about new heads
is quite helpful.

If you don't want to merge, the maint tree will become a separate head after the
first push --force, so on subsequent fixes there is no need to use --force again.

Comment to your 'clone -r <TAG>': with this the tag will not be transferred,
because the tag is recorded in a changeset that happens after the to-be-tagged
changeset. Therefore you might want to clone the whole tree and update to the
tag later.

With named branches you would use either 
$ hg clone -r 1.2.3_maint foo bar
or even better
$ hg clone foo#1.2.3_maint bar
(the later would prevent subsequent pulls from pulling in history of other branches)