I don't know about under-seat storage, but it looks like you could use a cable lock to strap a helmet and/or jacket to the passenger peg hangers, or run the cable under the center of the handlebars. Also, I believe Motion Pro, or some other manufacturer, sells a helmet lock that could clamp to the passenger peg hanger steel tubes with security screws. I have one of these type of locks on all of my bikes that did not come with a helmet lock. The lock costs around $25 retail.
Mounting rear seat luggage or a rear rack looks to be more problematical, though.
Finally, that silver fastener on the frame panel below the seat might be a seat release button. I wouldn't put an money on there being much room under that seat. It looks pretty tight under there.
