Thanks for the feature request!!! This will be a nice feature which will enhance Mass Email Wizard of MassMailer.
I will keep you posted if this will gets implemented.
I have used your Mass Email Wizard of MassMailer and i am pretty much comfortable with this feature. You guys did an excellent job in order to build this functionality. Here, i would like to ask you guys can we make Header and Footer of template Body Non-Editable.