Is releasing retainage for completed Task Orders permissible even if the contract is still in progress?

WIthout knowing more and whether you're a prime, sub, or owner it's impossible to tell you if if retention can get released in your specific situation. But, in general yes it is possible. More often than not, it's a partial release or reduction down to 1%. Someone knowledgeable in construction law can probably point you in the right direction and help write a request for retention reduction.

I would consult with someone on the decision to releasing retention. It's a blend of a risk management and legal issue--but mostly risk management.