What you should add in the "Admin -> Layout -> Custom Tags in HEAD section" is something like the following:
<link rel="alternate" type="application/rss+xml" title="SMSLib Q&A - Questions" href="http://example.com/qa/script.php"
<link rel="alternate" type="application/rss+xml" title="SMSLib Q&A - All Posts" href="http://example.com/qa/script.php?all=1"
I modified the initial script and now I use two feeds, one with only questions and one full.
Here is the new script: http://pastebin.com/3pgceAci
If you call it with no parameters, you'll get a Questions feed. If you call it with "all=1" you get a full feed (questions, answers, comments).
I don't use the script directly - I cache its output to static xml files and publish the static files.
Forgot to say that everything in the script in-between the lines "***CHANGE***" need to be changed to your URLs.