sectionpage field

It could well be that rotating the diagram would, in the long run, be the easiest approach. The alternative will be a REF field instead of SectionPages. Assuming that you are restarting numbering in each logical section, this will not be too difficult. Restart numbering as usual on the first page of the logical section, then continue it through the landscape section and the portrait section that follows. Insert a bookmark somewhere on the last page of the section (somewhere where it is not likely to be deleted but is certain to remain on the last page). Instead of the SectionPages field use a REF field referencing the page number of the bookmark.

You can insert the field easily using References | Captions | Cross-reference: Reference type: Bookmark; Insert reference to: Page Number.

Insert the bookmark using Insert | Links | Bookmark. The bookmark must be a single word; it can contain numbers but can't start with one. So you can use LastPage but not Last Page, Page2 but not 2ndPage, etc.