Optionally exclude book structure on menu block creation

Hello!
I administer several book-heavy pages (on Drupal 6, with hundreds of indices). By incorporating the book structure in the menu selector during block creation, this makes it impossible for me to choose a menu – I thought the hierarchy was just mixed up, but the regular menus items seem to be driven out of the dropdown selector altogether, I can no longer access them over the menu selector.
Currently the only solution for me is to is to turn off the book module, choose my menu and then turn it off again.
Would it be possible to make the book structure during menu block creation optional?

Thanks for any help on this!