Well I can't say for sure why they use it, its not any traditional outfit I know of but the band is called "velha gaiteira" which literaly means "old lady bagpiper".
From their website, they say Velha gaiteira is an ancient character of the Portuguese Folk, which, according to folk stories, was all the women who loved parties and everything that could gather the People, as, for example, music.
There are no mentions to the outfit but a lot of old ladies from small towns tens to dress more or less like that, so...
All the bagpipes played in the videos are Gaitas transmontanas in Sib minor as far as I can tell, the forth video just shows pictures of a few different styles of construction.
As for the sheet of baile das oliveiras.. I don't have it yet but I'll try to write it. Embellishments apart, its a fairly simple tune. Ill say something when its done
Actually, I found out today that the band that actually played the bagpipe part of Noidz's Root sounds of the earth where Roncos do diabo hehehe
As a curiosity for you, the main players of Roncos do Diabo are Vitor Felix and Mario Estanislau, the makers of my gaita hehehe