What is the recommended method to add an RSS icon to an Open Scholar scholar site?
If you have a footer menu, you could add the RSS feed to it, and then attach a bit of CSS to your theme.  The exact selectors would depend on what is being output, but you could do something like:

ul#footer-menu li.menu-1234 {
  padding-left: 20px;
  background: url(/path/to/rss.png) left center no-repeat;
}


You may need to adjust other styles to get the whole icon to appear.

There are bunches of sites with royalty free RSS icons.