Since the posts above I have done some further digging.

First, the CPAN module Win32::Env has a BroadcastEnv method. However there are issues, as documented above and in the Win32::Env documentation.

Second, I have written a module that sets env.vars using DLL injection which I will put on CPAN. /msg me if you are interested.
I expect issues on Vista for this, and I have a fix, but I don't have access to Vista just yet. I will not upload onto CPAN until I have tested on Vista.

There is also the possibility of getting (as well as setting) another process's env.vars. - that is an extension I am currently working on.

I will keep this updated.