The real question is why you need mandatory locks and not advisory locks. If everyone is playing on the same team, advisory locks should provide all the semantics you need (and are very portable). Mandatory locks (on linux at least) require not only special mount options, but special perms to the file (g-x, g+s, I believe). That seems like a lot to require inside an extension.
The dba solution so far is based on flock() and where not appropriate use fctnl().
I tried to have the lock stuff working on as many systems as possible.
marcus