If you want to solve the GPL compatibility problem cleanly, why not
dual-license? Then, you can use Larry's form of termination. If anyone
litigates against free software, the Apache 2.0 license terminates and
they are left with the GPL as their only valid license. Then, if it
happens that they had previously mixed proprietary software with Apache,
or they depend on any such software, that gets poisoned.
Thanks
Bruce