Suggested here

Comparator Output
What should determine comparator output?
Item frames use the rotation, but that only has one item allowed. Perhaps it could be how full the inventory is?

Redstone Signal Ouptut
Maybe have it be a setting configurable.

Which sides should it output?
Probably just the block that it is placed against with strong power and the 4 side faces as weak power? That would make it like redstone dust.

How to configure?
Probably a mode for the tool?