I searched and this hasn't been discussed in detail and not for years.
When I started playing indoor a couple of years ago my first instinct was that ping, the middle, and the quality of the willow were totally irrelevant, and if you've got something light that you can move quick enough to get a bat on every ball, that's all you could want from an indoor bat.
Having said that, there are sites like MSR and SQ sports which advertise specifically indoor bats, although I assumed these were just bats that were over dried or of such poor quality wood that you wouldn't trust them against a leather ball.
Is there something to look for in an indoor bat. I'm not suggesting I'm going to buy a grade 1 bat and smash it about the floor of indoor court but hypothetically, is a decent middle and good quality wood going to make any difference at all to those yellow foam balls?