I've figured out that the lens doesn't quite do what it does in newer zelda games. It does show maze paths and such, but there need to be more flags!
Possibly a screen flag-- "Lens shows secret combos"
A quest rule and screen flag-- "Lens on layers only" or checkboxes for those layers
A quest rule/flag -"new lens" for a OoT-style lens of truth/ "lens" flag
To follow the above, add lens to the secret combos.
These would be really convenient for a shadow temple-type dungeon