Not sure if you still need help on this, but in the LiveCycle Designer documentation ("Adobe XML Form Object Model Reference) there is a script in the examples called "Disabling all form fields" You can apply this same logic to subforms (and I suppose an individual field, I did with with subforms).
So you will want to have a hidden field in your form (or based on any field in your form) and depending on that value, set the subform to readonly. If needed, you can set this field using the SetVar QPAC in your workflow depending on the step you are on. My only problem is that I have my form as a document variable, so I am sure how to change my hidden field after the first step and my form is within a document variable. But, if you are using Form variables, I don't see why this shouldn't be a problem.
Hope this helps!
Jennifer