This is a flaw with simple pie. The automatic Feed locating mechnaism does not identify "text/plain" as a valid contenttype for a feed. Technically spoken it is not a valid contenttype.
Since the decision is based on contenttype, it could be added (as in the patch) to get it working.
WP should not fix it but SimplePie or there is another option, called force_feed which can be set with ->force_feed(true) in feed.php (second patch). That seems far more reasonable, since