The actual engine has a big sort of dial on it that the skink priest has to set at the beginning of every turn to determine the setting of the engine for that turn. As the dial is set well before the magic phase the priest generally has nothing to do in the magic phase except admire the magical pyrotechnics of the slann and avoid falling asleep from the gentle rocking of the howdah as the steggie ambles around. Therefore the priest could theoretically cast a spell as he has little else to do (except dieing quickly like the skink he is!), nothing prevents him and the EotG actually gives him a +1 casting bonus if you have any power dice going begging.