I think
Boromir, Bearer of council is working fine. If the text was "At the start of each skirmish involving Boromir, add 3 burdens. The Free Peoples player may wound Boromir twice to prevent this," then if you did not end up wounding Boromir twice, you would have to add the 3 burdens. However, that's not what is said. You are allowed to choose to wound twice, or to add 3 burdens: subtle difference. If you could wound Boromir twice (so not wearing
Armor), you may choose to do so. This is not a cost to perform an action (such as preventing the 3 burdens if the text of the card was as above), so if the wounds do not end up being added, no biggie.