I tested 3.4.d and 3.4.e, and in both there is a bug that makes RSS Dashlet doesn't load RSS feed coming from Joomla! powered sites.
I got two examples for differents sites so you can make a test. I submitted a bug and QA said it was fixed in 3.4.1 Enterprise, but I have no idea how to recognize if the fix ships also with Community releases.
Hey Jon, we spoke yesterday on IRC about this I think
I did some testing as well, and I think the bug is probably more subtle than you realise. Your RSS links didn't work for me, but I also tried changing your RSS links to Atom format, and that also didn't work for me: so I don't think it's the RSS parser per se, it has to be something slightly higher up the stack perhaps. I have a couple of Atom feeds on my dashlet which work great.
My guess is that there is some difference between our stacks and the ones Alfresco were testing on which doesn't show up this bug. I don't think it's fixed.