How about creating a third Details section and putting the subreport there? Set the property New Page Before so that it forces a page break before printing.

For the page header in the sub-report, you have to fake it by creating a dummy group and putting the header/footer info there. Set it to print on each page.

Ah yes, of course it would be inserted in with the other detail sections. My mistake. You could put the subreport in the report footer to make sure it ends up printing after all the detail records. If for some reason that conflicts with other fields in your report footer, you can create a fake group that goes around the detail a and b sections and put the subreport in the group footer section.