This workflow may do what you want:
1 - launch iPhoto with the Option key held down and create a new library.
2 - connect the iPad to your Mac with the new library open.
3 - import all of the photos from the iPad into the new library.
4 - select the option to delete all of the photos from the iPad.
5 - disconnect the iPad and delete the new library.
6 - launch iPhoto with the Option key held down and select your original library.
7 - assemble the photos destined for the iPad so you can easily identify them by album or event which you alread have done.
8 - launch iTunes and connect the iPad to your Mac.
9 - go to the photos section for you iPad and synce those particular albums or Events with the iPad.
That will get those specific iPad photos back into the iPad.