but they haven't yet wounded a ringbound companion... Just like Boromir, BoC, he can't wear the ring to prevent the two wounds.
You are correct about
Boromir, BoC not being able to take those 2 wounds as burdens. He actually can do it, but the cost is not fulfilled and so he will have to add additional three burdens. But that is the cost, not the effect and I think you had those two mixed up a bit.
You see, when
Boromir, BoC as a ringbearer starts a skirmish phase, he has to pay a price for being the Ring-bearer. That price is either 2 wounds or 3 burdens. If he's wearing The One Ring at the time, he can't add 2 wounds and must add 3 burdens (same would happen if he was wearing
Armor at the time).
In the case of
Howl of Harad, wounding a ringbound companion is an effect and there is no
OR here. If it's prevented, it's prevented... that's it. (or turned into a burden, for that matter).