You support Ruby.
You support HTML.
You support ASP tags in HTML.
You have all of the prerequisites.
A RHTML file is simply a HTML file with Ruby code
embdedded in ASP tags.
<p>Whatever goes here</p>
<% ruby.statement %>
<%= myvar %>
<%# The above one returns the value of myvar %>
<%# These are comments %>
This should be /really/ simple to implement. All you'd
have to do would be to detect .rhtml/.rxml files and
instead of ASP code inside <% %>, highlight Ruby code.
And then understand that <%# text %> is a comment.