The question was whether the ABS unit would be damaged by fluid being forced through it in the wrong direction. It had absolutely nothing to do with air in the lines.
Also, this is the first time I did my Mustang. I have done many of my Hondas and I just used my bare hands to compress the piston back in
no tool needed...
ONCE AGAIN I will post the text that I already posted from the Bullitt Archive:
NOTE: Just to be safe when compressing the pistons back into the caliper, instead of forcing the fluid backwards through the ABS unit and the Master Cylinder, first install a hose on the bleed screw, so it can drain into a jar and then loosen the bleeder. This way you are just pushing the fluid out instead of backwards through the system. Don't forget to tight the bleeder screw after the pistons have been seated.
Forcing the fluid back through the ABS unit can damage some types of systems. I was not able to verify that ours would be damaged but why take the chance.
I have seen that happen a few times. it's not that he did not know, it's that he forgot. little stuff like that becomes so routine that your like. wait did I just do that or did I forget. 
