Just found this, what I assume to be an oversight and comical bug, easily replicated so not posting a player log.
For simplicity sake going to use the werebear and its rampage skill as an example, but this seems to work in all druid transformations with all transformation skills.
1: Transform into wearbear
2: click on the hotbar slot you wish to bind rampage to.
3: Hover the mouse over rampage in the selection menu
4: transform back to human form and then click rampage.
5: Use rampage while human with zero resource costs.
It doesn’t appear to consume rage or mana to use the skills.
Weirdly enough it is still affected by transformation scaling items and crafts.
Links below show werebear skills on my hotbar while human and casting maul while human.
http://prntscr.com/22s0eu9
http://prntscr.com/22s0fqn
Edit:
experimented further and this bug has some limits you can use spriggan skills as werebear, swarmblade skills as spprigan, etc… you can assign any skill from any form to any other forms hotbar the only exception being taking a human skill into a transformation.
But you can only take one skill from one form to another as any skills that don’t belong to that form will be reverted back to the skill that does belong that was bound there.