Assigning individual particles to fields (runtime)

I have a particle object... assume all particles are within the field area. I need to flag individual particles so that they become affected by the field after a certain point.

(Imagine bubbles forming on the inside of a glass of coke, eventually they are released, and rise to the surface...)

I'm using opacityPP as a flag for now (cos Im rendering blobbies so its spare) I use this set to 0 at creation time, then after a certain point I set it to 1.0

There will be no dynamics on each particle until the flag is set, if thats any help - but I am changing each particles radiusPP and positionPP individually up to that point.

The best solution would be linking the particle to a given field, but linking the particle to dynamics would also do.

Anyone got any suggestions?

Cheeeeeers!