Yes, it's possible.
It will require some programming skills though. People make this kind of software all of the time - primarily for spamming forums.
Your case is a little simpler since you don't need automate and get past the sign-up process.
You can use any programming language that is capable of speaking HTTP i.e. just about any language out there today (it doesn't have to be PHP). Indeed, a desktop client (software that runs on your PC) is probably better for your purposes.
Basic Instructions:
Simply make an HTTP POST to the following URL:
http://www.elitetrader.com/vb/reply.php?action=postreply&threadid=XXXXXX
Where XXXXX is the id of the thread you want to post to.
You will need to have form fields that match those found on the reply page e.g. title, message etc.
Oh, you will also need to do an HTTP POST to login to your account and handle cookies.
You can use an off-the-shelf screen scraping tool to do the above or to at least record the request/responses when you login and post which should get you started.
Or, you can get someone to write it for you.
Try somewhere like rentacoder.com or elance.com or scriptlance.com
Shouldn't have to pay more than $50. 2 hour job.
If you want it hooked up to your trading platform, that's extra work but again, not that difficult depending on the trading platform/API/hooks available.
[EDIT] Sorry, just read your post again...since your platform writes to a file/files...you'll need a file watcher program as part of the equation. If you could get your platform to send e-mails instead that might be a little easier to hook up either way...it's doable.
So, you could have the software post to ET automatically when your trade is filled.
If you do a search, there's a good chance that there is already software written for posting on vBulletin that you can use.
As per the other responder, vBulletin, especially the version that ET is presumably running on, pre-dates the times when web services/APIs became common and as such, aren't really set up to do this kind of thing as seamlessly as Twitter et al.
DISCLAIMER
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Doing this sort of thing might be against ET policy but to be honest, I think ET should develop this kind software and give it to people posting in the Journals forum.
The ET software might also be set up to block this kind of thing automatically by checking referrers, tokens etc. in which case the whole discussion is moot and it is not actually possible to do.
