I don't have an answer, sorry, but maybe I can inspire you to think of one?
Re-phrasing the statement in the positive, I assume 'Reduce the size of the enclosing part' could say in addition, "...but only if it is a body, sub-summary or grand summary."
Can you come up with a usually-nonsensical break field to artificially create a Leading Grand Summary, Sub-summary (leading), Sub-summary (trailing), and Trailing Grand summary that could, for your particular purposes, be made to substitute for the various header and footer parts?
Bit of a wild shot, I know,
The Header and Footer information is something that is fixed and appears on every page. Think of it as a word processor or spreadsheet header/footer. Maybe the better question is, "Why do you want the Header/Footer variable length?"
"Sorbsbuster" also makes an excellent comment (Thank you!). That is, the Body, Sub-Summary and Grand Summary are all variable length. Does this help?
Yeah, I get the idea. If it's really important to me, I may figure out a way to use the other Layout parts to handle this.
Thanks, as usual.