Perhaps a set of wireless burglar alarm "contacts" in the tank, fastened in such a manner that hitting the flush will " trigger the wireless contact" and in return will send a signal to its receiver elsewhere, and from there control your light flicker circuit. Heck for laughs it can even momentarily sound a big 'ol bell! Wouldn't that be " alarming"
Figure that may be easier than plumbing in a flow switch, and best off, it will come out as easy as it went in when the joke is over.
A.D