Planking is the worst strategy in SSB64 unless you're Samus, and even then it's not the best idea since nearly every character needs to expose a good chunk of their hurtbox above the ledge (easily hit by most smash attacks) in order to be in grabbing range. Not to mention the character must be "falling", so that's even more time you spend dilly-dallying above the ledge.
To my knowledge, there isn't any cooldown time, but that might just be because you physically can't grab the ledge enough quick enough in rapid succession. You have to let go, wait for Ness to do his stupid double jump dip and then start to rise before you can PSI Magnet cancel. And all the while, all your opponent needs to do is d-smash and all of a sudden you're in the worst position possible with Ness.
You have quite a few frames of invincibility immediately after the grab, but I think you lose it the moment you let go. Your inability to view the effect is due to your graphics plugin; try using Glide64.
Repeatedly grabbing the ledge is a cool concept (and that's all it is), but Ness can't do it fast enough to be cool enough, haha. Not sure about Yoshi's magical double jump grab cancel thing, and if you can just let go and do it repeatedly, because then you could do that damn fast, but I think you have to be in a position that's higher than falling off the ledge for that to work, not to mention its strict frame window.