i know that for the 700 you can embed fonts using epub without any noticeable loss of speed. i believe you can embed fonts in lrf as well but it causes the file to be very slow to page through.
this thread explains how to specify fonts using css rules, which can be added manually or in calibre. it is for the 700 but it might work for the 505 as well. let us know if you try it out, what result you get.