A really nice feature but I would not want my users to have to install P4Python by themselfes.

I don't know whether you want to add a dependency to P4Python although this could also be done using "p4 change -o" and "p4 change -i".

Since you're only using this regex once, you can use re.match as a shorthand.

Two comments here:

1. It would be really nice if this could update an existing 'Review Url:' field. At VMware we have changeset templates with a bunch of fields in them; being able to add an empty field to the template and let post-review populate it would be awesome.

2. A more pythonic way of writing this is:
    if 'Review Url:' not in desc:

This won't work now because execute sets shell=False. It might be helpful to add some plumbing to plumb some stuff through to execute() such that you can dump the file into the pipe for stdin. This would also eliminate the need to write the file out to disk.