I don't think it's very handy to link in an external website. It might be rather confusing then. I'd prefer that everything stays within Opensubtitles. As for implementing this feature, I suggest the following:
If a user searches for a subtitle and gets no results, and if there are already requests existing for this movie ID, he'll get a direct link to those requests, so he can instantly see the status. There he can: A) add his own request or B) set a flag that he is working on that subtitle
If a subtitle has been flagged as 'Subtitle creation in progress', it will be displayed to users at the search results. If there are no results, but if a subtitle is in production, they get a clear hint about this and also a link to the subtitles existing requests.
In order to make this useful, the requests lists' functionality should be extended. Subtitles producers should be able to post info on the production status, e.g. estimated finishing time, percent of completion, comments, help/support requests for their production. Perhaps this can be simply done by linking it to a subforum of OS, specifically for discussing the production of those very subtitles (sharing the work, asking for translation parts, etc). This subforum should have a 'one thread per subtitles' policy (movie id + language) with the IMDb ID and the language being compulsory in the thread's subject title.
If a subtitles file hasn't been submitted within a specific period of time after it has been flagged as 'In Production', the flag gets automatically removed. Of course, subtitles producers can simply 'renew' it by setting the flag again if they need more time.
Naturally, only registered users can set this flag. They can also unflag it, if they don't find the time to finish it. Unfinished subtitles could be posted as attachments in the subforum, so that other subtitle producers can pick it up and finish it if they want to.