The father of my children owes $27,000+ in back child support. He now has custodial care of our 17 year old children. Even though there is no court order for me to pay child support, he has filed for a lien against any "future child support" I may be ordered to pay. Is this a viable legal maneuver to reduce his arrears?

Yes, So lets say he is awarded $200 a month until your child reaches 18 lets say that amounts to $1000. So his arrears will be reduced by $1000 if the court agrees.

He can't take what isn't there. If there is no order then he can't place a lien.