This is a pretty gutsy move by Estonia if they do that. Estonia is a small country with most of its border towards Russia. They have little chance to defend themselves if Russia is stupid enough to decide to invade.
Obviously being a NATO member is the best protection there is for Estonia, but does chapter 5 still work, if Estonia sends personel officially to Ukraine to participate in the defense of Ukraine, even if it isn't on the front?
I hope we will see this happen soon, I am pretty sure that if Estonia does it, other countries will follow. That's the way it's been with everything else. If one country takes the lead others follow.