I understand. This functionality was intentionally left out because of performance issues. Custom fields can make a huge difference. However if you want, you can still make it work.
I've made a code change, so it might work:
1. Open up the wp-content/plugins/ajax-search-pro/includes/search_content.class.php file
2. Make a backup of it's content.
3. Replace it's content with this: http://pastebin.com/raw.php?i=iQTh8jKc
4. Save it and try it.
I'm not sure if ti will work, but it's worth a try.Best,
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