well my idea for arms-backpack is that the player carries the backpack on it's back and the only new animation for the player is that he pressing some random buttons or pulling some lever infront of him (which is attached to the arm-pack) and the device does some "random construction animation."
my clockwork idea is a summoner/dps kind of class (similar to engineers in TF2 and similar turret builder classes) they put small turrets operated by a clockwork-motor. and tinker uses these turrets to cast it's abilities (he himself does not cast the abilities but orders his turrets to for example cast AOE CC on X position) the tinker himself uses his equiped ranged weapon (gun prefered) and some basic ranged abilities to deal basic dmg but the majority of work is done by his small but hardy turrets. he can also build stations, (these creations are miniatore and do not take big spaces, a turret is the size of a totem and stations are a bit bigger) stations do some other stuff like allowing Tinkers to detect stealthers around him by casting "radar scans" on the position Tinkers choose. all these creations have limits of course.
so as you can see the mechical arm does not require much animation in this model, just a construction animation (mechano arms aimlessly moving in the air and opening closing their hands) perhaps a hitting/slapping animation is also needed :P