I was looking for some easier solution than using web forms, but no minilab seems to support Linux here. Windows applications I tried so far does not seem to work in Wine, so this is also not an option. Only thing that seems to work is Picassa, but it is quite big tool to make just uploading photos and it is anyway some beast using Wine.
So the only option was to write something myself. Using cURL from Python is quite easy so I hacked simple script to upload photos to minilab using PycURL. It currently supports only Droxi, but it might be later extended to support others. The code is also now a bit fragile as it does almost none error checking, but I was able to upload first set of photos to minilab and continue order in browser.
You can download my proof of concept code at Photo uploader page, but don't expect anything clever from it, but it should basically work :-).