Hey, I like where you're going so far with this. Since you mentioned you wanted to have some explosions and effecs and such, heres some ideas I was playing with a while ago:\
Mortar/shell hit:
First I just crumpled tin foilo into a basic shape, and poked in toothpicks where I wanted the little things "shooting outwards". Last I just covered in plaster of paris, I think wall spackle would be jsut as good or even better. this was going to be a mortar hitting water a Omaha beach, so for an urban setting, throw some bits of debris or even weapons and rebar and stuff coming out of the explosion. Last, you could use some wool or stuffing painted appropriately, and glue sprinkles of sand on to it to show the dust and dirt getting kicked up. If you threw an orange LED there and paint4ed the mortar hit niely, that could be one awesome looking explosion.
Rubble:
I know wrubble isn't hard to make, but this was pretty easy. I just poured the extra plaster into a CD case, and let it harden before bending the CD case backwards would made them crack into nice little squarish chunks.
I ahd a technique for ullets hitting and kicking up dirt, but I think it would be too big in 1/72 (the two above were both 1/35, but could easily work for any scale.
Hope this helps!
"Scanlon: work your knobby hands on the table in front of you, constructing a make-beleive bomb to blow up a make-beleive world."