Is there ANY way to activate a planeswalker ability at instant speed?
Since the other answers have been written, a card has been released that explicitly allows activating loyalty abilities any time you could play an instant: Teferi, Temporal Archmage from Commander 2014. 

He's a planeswalker with a relevant ultimate:


  −10: You get an emblem with "You may activate loyalty abilities of planeswalkers you control on any player's turn any time you could cast an instant."


So if you manage to get him to 10 loyalty and activate this ability, you will be able to play loyalty abilities of planeswalkers at instant speed. It's not an exciting combo or abuse of the rules, but it can still be a lot of fun in a slow multiplayer game—exactly what the whole Commander 2014 set is all about!

As for activating loyalty abilities multiple times in a turn, that is possible with The Chain Veil.  A card first printed in 2015, long after the question was originally asked.