All StudioLive mixers use the same basic routing. so on a Series III mixer I would do it like this:
1) Aux In 1 (stereo) as input for iPad music, sent to
2) Aux 1+2 (linked) as "bus", which gets
3) routed as stereo input to Capture tracks 17+18.
This eats no inputs, just a stereo aux. I could use a "fixed" subgroup on the Series III mixers as bus for this, too.