How do you handle files that can't support concurrent edits in Mercurial?

I'm using Mercurial with TortoiseHg. Each developer has their own repositories, and there's one central repository on the server for synchronizing our changes. (This will sound lame, but we're using it to manage the source for a legacy VB6 project. Nothing we can do about that...)

As has been pointed out elsewhere, there is a big problem in VB6 with merging the .frx (form resources) files. So code changes seem to merge fine, but if two developers both make changes at the same time in the form design view, we can't merge.

I'm ok with disallowing concurrent edits, but of course the whole point of Mercurial is that it's distributed so there is no option to force a file to be locked before editing. I don't believe there's a Mercurial solution for this, so I'm wondering: other developers who are using Mercurial for version control, do you hav开发者_运维问答e some 3rd party tool that assists with locking files for editing in the cases where it's necessary? Did we make a mistake using Mercurial instead of something like SVN?

Heard of some people using a standalone lock-server (this one in particular).

This is from Bryan O'Sullivan's book on Mercurial:

There is no single revision control tool that is best in all situations. As an example, Subversion is a good choice for working with frequently edited binary files, due to its centralised nature and support for file locking.