Here's a very simple way to do it for DateListPlot:
So the OC could actually rewrite DateListPlot to add error bar support, if he wanted to. He could even overwrite the original MX file in his installation once he had done this, if he so desired.
Even thought it's true, probably he couldn't legally distribute his own modified copy of DateListPlot. Right know you can hack together error bars for a plot even if you only know the output format of DateListPlot (FullForm and patter matching helps a lot).
Thanks for "Spelunk", it's awesome.