I have seeing this question all over redit and other forums so, after some testing I found active spells can be cast via rune slot:
/cast Legs Rune Ability
/cast Hands Rune Ability
/cast Chest Rune Ability
There are some odd exceptions, for instance Mangle Replaces Claw.
Ergo, if you have /cast Claw, it will cast Mangle in cat from if you have the Mangle rune equipped.
This also seems to resolve the Sunfire vs Moonfire (Rank 1) macro problem in form.
Hope this helps everyone!