Can't really see that the extra weight of a Euro-sceptic politician would influence things that much. The Wilga at our club routinely tows half a ton of Grob Acro (all up weight), accelerating from a standing start pretty smartish, and climbing perfectly happily thereafter at 4-500 fpm. So 75 kg in the right hand seat isn't going to make much difference with a beast like the Wilga.
The reason passengers are not allowed when banner flying is presumably not due to their extra weight but due to the enhanced level of danger of such flying escapades.
Remember the case of the Air France A320 that crashed in to a forest at the other end of the runway while overflying a General Aviation airfield (where the A320 was not actually able to land) at low altitude with a full complement of passengers on board. Passengers were forbidden from being carried on such exhibition flights involving commercial aircraft as a direct result of this accident.