Catalysis of the reaction: tRNA-(2-methylthio-N-6-isopentenyl adenosine) = tRNA-(2-methylthio-N-6-(cis-hydroxy)isopentenyl adenosine) + O2. 2-methylthio-N-6-isopentenyl adenosine is also known as ms2i6A; 2-methylthio-N-6-(cis-hydroxy)isopentenyl adenosine is also known as ms2io6A and 2-methylthio-cis-ribozeatin.