Doesn't seem to be a bug, and doesn't seem to be a problem; Mundo/team mates need to move in for the kill, and it takes a while for Mundo to whittle Nashor's HP down to a manageable size before anybody can kill it with relative ease.

^ So it shouldn't be exploitable; it does %Current HP, so it'll take quite a while to get Nashor down to size.

In fact, going to test this out now, BRB.

Edit: just tried it, takes like 5 bloody minutes to get Nashor down to 1000HP, and then you sit there in limbo because Nashor's HP regen is fast enough to counter your Cleaver every 3 seconds. I'm not sure if a team mate initiating Nashor at this point will reset Nashor's HP, but yes, the very fact that Nashor can't retaliate (a la Tristana exploit) is rather dodgy.

You can forget about exploiting this in-game though, it's really boring, and takes ages (I was level 5, so I was using level 3 cleavers, still took ages), and enemy team would most likely kill you, or worse, ks Nashie.