Thats how I use to grow them several year ago. When I was aquaculturing zoas I mounted them to ceramic floor tiles and would place the tile vertical and they seemed to grow much faster. They would always grow toward the light.
+1 feeding dose wonders and i also have some zoas that I threw onto a vertical service which are growing like crazy even without feeding. Also have seen this on frag racks were the colony will grow more polyps from underneath the frag rack up towards the light, kind of a pain to get them off but nice to frag the colony from underneath rather than the top portions =P