>> I can provide a patch for this. 

Please find attached the patch.

> Since "ox-rss.el" loads "ox-html", could you re-use code in the latter?

I think even ox-html could benefit from using an XML/HTML generation
library that can convert s-expressions to XML/HTML. xml.el has
`xml-parse-region' to convert XML strings to an s-expression tree, but
unfortunately has no function to do the inverse, that is, convert an
s-expression tree to an XML string. If it had such a function, we
needn't have to depend on an external library such as xmlgen, and things
would have been more convenient.

Anyways, for now, I have implemented a patch which uses
`org-rss-plain-text' (which, in turn uses `org-html-encode-plain-text')
to encode the disallowed characters to their XML entities.