I've just release rAtom 0.3.0. This version adds support for simple extension elements and also checks that content is in UTF-8 before serializing to XML.
As defined in the Atom Syndication Format, simple extension elements consist of XML elements from a non-Atom namespace that have no attributes or child elements, i.e. they are empty or only contain text content. These elements are treated as a name value pair where the element namespace and local name make up the key and the content of the element is the value, empty elements will be treated as an empty string.
To access extension elements use the  method on the Feed or Entry. For example, if we are parsing the follow Atom document with extensions:
Feed with extensions
We could then access the extension element on the feed using:
> feed["http://example.org", "myelement"]
=> ["Something important"]
Note that the return value is an array. This is because XML allows multiple instances of the element.
To set an extension element you append to the array:
> feed['http://example.org', 'myelement'] << 'Something less important'
=> ["Something important", "Something less important"]
You can then call to_xml and rAtom will serialize the extension elements into xml.
> puts feed.to_xml
<myelement xmlns="http://example.org">Something important</myelement>
<myelement xmlns="http://example.org">Something less important</myelement>
Notice that the output repeats the xmlns attribute for each of the extensions, this is semantically the same the input XML, just a bit ugly. It seems to be a limitation of the libxml-Ruby API. But if anyone knows a work around I'd gladly accept a patch (or even advice).
You can get rAtom via gem:
> gem install ratom
or from Github.