"k.i.s.s keep it simple stupid" or in other words "do not outsmart yourself".so when i heard of pop-up shops, immediately pop-up concerts came to my mind.
in the discussion people changed it to jazz-up.
now for the real action, i would more prefer the term hold-up as used in the wild west genre.
the ideal situation is, to show up and play. in reality this only is feasible on jam sessions.
new as of june 2011: make it jazsoup, it is more mixed, mangled and boiling.
my translation of jazsoup into german is "ein marmaladebrot" just in case someone reads this or that.