if you had the time to learn it well enough i think the ideal encore piece, bearing in mind your programme , would be a BRAHMS HUNGARIAN DANCE. numbers 5 or 6 are household famous pieces, so i'd recommend one of those to please the non-classical listeners. number 1 is my personal favourite but maybe not quite as famous (also it's harder). they are quality virtuoso pieces which nobody could accuse of being banal. I think it would follow on perfectly from your programme, after your Brahms and then the Liszt with the Hungarian flavour.
they are not easy however (i'm learning them for concert right now), which is why i express reservation about how much time you have. i guess it depends how fast you learn. if you are doing the Hungarian rhapsody no.2 you obviously are at the right level, the difficulty is very similar although the dances of brahms are shorter, 2-3 mins which is the ideal length you are looking for. No.5 in f# minor is the most famous and the easier of the two, also very easy to memorize, but the challenge is hitting the right notes at full speed and dynamics!
good luck whatever you choose, i think your recital programme is very cool by the way

do post and tell us all how it goes