Looking into this myself. Believe you can go longer than the 'normal' 12" - 15" BUT would need to change design somewhat. LisaLou mentioned rotating hooks on BOTH ends which I believe to be key but no swivels can be used.
Rotate from BOTH ends to keep twist rate more uniform along brush.{Believe Gunnar Brammer does this} Rotation need of course be in opposing directions.
An option would be to fix one end and have the other on embedded T-Track to adjust working length.
Use 2 or more drop-down sections to allow for making different length brushes.
Spring should still be necessary on only one end but definitely needs longer 'travel' than those built for shorter brush.
Believe opposing twist rates would allow much longer brushes when wanted keeping twist rate closer along brush length.
T-Track would allow brush length adjustment for different types from light trout to massive musky/saltwater sizes.
Drop table(s) made of multiple sections to allow making shorter brushes when wanted/necessary - a 12" & 18" allowing 12"/18"/30" a suggestion.
The longer spring travel may also need to be stronger.
For those who find them clumsy to work with longer brush can be cut to whatever length you desire once twisted.
