I would expect that requiring a notice of non-endorsement would not exclude a license from being a FLOSS license, considering that the original 4-clause BSD license is a FLOSS license despite its "obnoxious advertising clause":
- All advertising materials mentioning features or use of this software must display the following acknowledgement:
- This product includes software developed by the University of California, Berkeley and its contributors.
I don't know of any extant license that actually does require such an assertion, though.
The Creative Commons licenses already handle much of this, but not exactly how you request. CC licenses don't require a declaration of non-endorsement, but they still
- disallow a positive statement of (or implication of) endorsement, and
- allow you to have your attribution removed in any cases that you find distasteful.
The full relevant FAQ item is:
What can I do if I offer my material under a Creative Commons license and I do not like the way someone uses it?
As long as users abide by license terms and conditions, licensors cannot control how the material is used. However, CC licenses do provide several mechanisms that allow licensors to choose not to be associated with their material or to uses of their material with which they disagree.
First, all CC licenses prohibit using the attribution requirement to suggest that the licensor endorses or supports a particular use. Second, licensors may waive the attribution requirement, choosing not to be identified as the licensor, if they wish. Third, if the licensor does not like how the material has been modified or used, CC licenses require that the licensee remove the attribution information upon request. (In 3.0 and earlier, this is only a requirement for adaptations and collections; in 4.0, this also applies to the unmodified work.) Finally, anyone modifying licensed material must indicate that the original has been modified. This ensures that changes made to the original material--whether or not the licensor approves of them--are not attributed back to the licensor.