I don't think that it'd be supported but as a new feature it'd be nice to be able to assign weights to post types much like assigning taxonomy weights to a custom relatedness query.
10 posts in post type 'something_news'
3 posts in post type 'something_crossword_puzzles'
5 posts in post type 'something_classified_ads'
if I do a relatedness query I can force a taxonomy, and weight a taxonomy, but I can't weight a particular post type.
in the above example if I'm looking for related posts to a 'something_classified_ad' it would make senese that I could weight the other posts in 'something_classified_ads' higher than posts in 'something_crossword_puzzles' even if the title, content word match or the taxonomies were matched.