Using Mecanim to only change state on last Sprite of Sprite sheet

Hi
Trying to use Mecanim for something it perhaps it wasn't designed for.

Instead of just a walking cycle and an idle cycle, I also have a client supplied walking to idle transition!
instead of
walking > idle

we have
walking > walking to idle > idle

So instead of letting Mecanim change state on any arbitrary Sprite or frame in the Sprite sheet, I can only allow a state change on the LAST frame or sprite so that it matches the first frame or Sprite of the next sequence.

Is this possible somehow using Mecanim? Perhaps some additional scripting required?
I haven't been able to figure out how yet so any feedback appreciated!

 

On the walk cycle/transition confirm "has exit time" is checked. This will force the walk cycle to end before transitioning into the next cycle/state.
Also review - has exit time - in documentation for further knowledge.