So, there's no real way to do this reliably with Google Analytics. The closest you can do is the following:
Create a unique Google Analytics account.
Paste the Google Analytics code on a page somewhere.
Using Firebug, Webkit inspector, Fiddler, or some other HTTP inspector, find out the value of the
__utm.gif image request that your Google Analytics snippet generated.
Embed that image on your forum post.
Now, none of the resulting data will be reliable except 'Pageviews', and perhaps location. All of the data about computer configuration, source, etc. will just be shown as it is on the computer on which you initially created the original