Too many variables!
I love tracking down the solution to these kinds of problems but, there are too many unavailable testing grounds…
My advice (coming from an RGB only kinda guy) is to find the Applescript guru associated with Block Graphics (there has to be one somewhere, even if it’s one of their clients). Their instructions on the Upload Policy page, http://www.blockgraphics.com/BBS/faq.html are pretty specific and not something I would want to experiment with using Applescript. (note: the usage of the adding ‘w’ to the file name, might be key if you do use AS; and, the no binary clause) They probably have some customized code (possibly javascript) that deals with stuff like the upload on the form page. Even if you did find a way to communicate ‘properly’ with the cgi at the other end, more than likely (with the level of security I saw on the site), you would be refused on the basis of not being one of the ‘allowed’ domains capable of posting to the cgi (e.g. from their ip not your ip address). This is a reasonable security measure on their part.
So, without their help you could possibly have a functioning script and not know it!
I’d communicate directly with Block to find out if this is even possible.