If you make a periodic timer, it just resets the animation every time it starts, if it's to fast the animation never finishes, but otherwise if a unit receives an order, he'll just change his animation. You'd need to make a trigger for every order ID for that unit, and order it to change it's animation, in addition to a periodic trigger. But, that wouldent make the animation go through it's full cycle... Lol, you'd need a global variable to count how long the animation has been playing...
Well, as far as I know PauseUnit freezes and preserves animation, you could use that in place of a stop order for every type of order, just pause and unpause. Then the periodic trigger would take care of sending and looping the animation. That's probably the easiest way to go without using jass or unit replacement.