You can use hg reflog (from the journal extension) and hg reset <hash>.
hg reflog -v
should give something like:
<old-hash> -> <new-hash> <user> <timestamp> commit --amend <some-path>
if that is the amend you want to revert, just use:
hg reset <old-hash>
The commit will be reverted to what is previously was and the changes that were amended should now be uncommitted changes (check using hg status and hg diff).