I'm currently in a situation where I have multiple tables in a book, and each table bridges across its spread. The index entry I want to make is on the right-hand (recto) page but, because the upper-left corner of the text frame containing the table is on the preceding left-hand page, the index entry takes the number of that preceding page, not the correct right-hand page.

In practice, this means I have to go back and manually change the index entry page number to one page higher - for every single entry.

Implementing this would just require another option for the "Type:" drop-down, with an entry field for the number of pages to add or subtract from the automatic entry.

I would see this operating as selecting "Current Page +/-" from the "Type:" drop-down in the "New Page Reference" dialog, and then typing the desired offset ("1" in my case) into a "number of pages" field.