You can do want you want with a combination of javascript and php.
What you do is get your user to include a piece of javascript where the src= is the location your php page, just set the php page to output:
doucment write (" whatever you want to appear in the way of products including html code ");
To see this in action take a look at my site
http://www.blogfuel.com Make yourself a bit of code and stick it on a page and see the result. Then enter the url part of the code into the browser address bar and see what comes up then.
There is a little bit more to it than this as you have to escape a few codes when passing to the doc write, but that's the gist of it.