conditional atom property

Has anyone ever made an atom property (or assigned it to a region, changed its type, etc) based on a conditional statement?

For instance, if I set some threshold of particle force, say...
variable crit_force equal 50
then i find per/atom values of force magnitudes
variable mag_force atom (fx[i]^2+fy[i]^2+fz[i]^2)^0.5

from there I'd like to say something like...
if "${mag_force[i]} > ${crit_force}" then &
[set the atom, i, into a group, change the atom to type 2, etc.]


but of course, to do it in this way, i'd have to make a loop over particles. But if the loop is done on the script level, it will slow down the simulation a ton.

The same could be done for monitoring per-particle temperatures and making changes, or many other things.

This seems like a feature someone would have wanted in the past, so there might already be a fix to help out, but I've come up short with my searches. Discussions on it would be appreciated.

Regards,
Jon