Item Group On Change OR Description Item custom fields

I have a new set of scripts that process estimates based on a list of description items in an item group.

I have a Boolean transaction column called included.  The idea is someone can raise an estimate an check the included box of a description item for the extra bits needed and these are used to seed server side scripts. But will never impact the accounts.

I want to default check the included box for a number of the description members of the item group. and i have two ideas:

1) Fire some client side script on the estimate.
I've looked at Field Changed and Post Source, but for item groups these both seem to fire before the members have been added.  I've no problem iterating through the list but I need to do it when the group has been added and exploded.  And it seems the members do not fire a Field Change Event when they are exploded.