For cumulative damage, why not use the current rules for damaged locations:
Page 46 Core Rulebook
If a location is hit more than once, roll for damage normally but the second and following hits add a cumulative +1 to the roll. Any result above 6 should be treated as a 6.
The damage bonus is permanent and relates to the exact hit-location that has been hit.
Would still work with these tables.
With regards to hitting a destroyed location, I would say to look at the rules on page 46 again, but there are two sets of rules that contradict each other.
Firstly it says:
Pick a location and roll the scatter dice. A shot that deviates to a blank or already destroyed location, or a location covered by an obstacle is considered a miss.
Then below it says:
If a location has been destroyed or blown off, the shot will hit the hull (if that wouldn’t make sense, choose another logical location) with an additional -1 TSM.
Odd
Anyhow, how about this.
If you hit a location that is already destroyed, consult the damage table. If the current destroyed status is not the highest result on that locations damage table, then immediately apply the highest result without rolling any dice.
If the destroyed status is the highest result on the damage table, then roll for scatter (again) to see if another location suffers damage from additional flashbacks or explosions. No saving throw is made for the hit on this location (go straight to the damage roll). If the second scatter brings up a location that is blank (i.e misses), then the resultant explosion has not been directed towards any other of the titans systems.
What this basically means is that if you hit a destroyed location, if it can be, then the location will be blown off. If the location can't be blown off, then you get the chance to damage an adjacent location, but if the destroyed location is on the edge of the template, the resultant secondary explosions might just vent to atmosphere rather than hit something further in the titan.
With regards, to hitting blown off locations, I would go with treating it as a hit against an adjacent location of the attackers choice.
Matt