A trial-and-error fix you can try is moving them all 1 pixel ahead and rotate them 1 degree every tick, then raise those values until you get the effect you want.
Otherwise this page explains the math a bit for rotating a point: http://www.siggraph.org/education/materials/HyperGraph/modeling/mod_tran/2drota.htm
(uses radians, so you'll need to multiply by 180/pi). It'll take a lot of tweaking though I think
Instead of moving objects to the "origin", you can run loops to add up the number of objects to a variable, the sum of their X positions to a variable, and the sum of their Y positions to a variable. Then calculate the center point between them as XSum/NumObjs and YSum/NumObjs.
Edit: Ah actually there's an even easier way to do that! Have a layer separate for the objects you want to rotate and then move them into it and change the layer angle instead
It's only a visual effect this way it seems, as objects go right back to where they were on other layers: https://dl.dropbox.com/u/4714446/LayerRotate.capxJayjay2012-11-29 15:56:53