Yes, the Pokemon will gain EVs. This also applies to Power Items.
To test this, I attached a Macho Brace to one Gible, a Power Band to another Gible, and had a Gible with no item. After defeating four Altaria with a Magnezone, the Gible with items had more Special Defense EVs than the one without an item (I used Super Training to compare the EVs).
I also tested this in ORAS, and the results are the same, although my way of testing was different.
To test, I used my Whismer. I gave it the Macho Brace, defeated 3 Mantine with my Manectric, and used a Grepa Berry on Whismer. Whismer still had EVs. I reset it's EVs and repeated the process with Whismer holding the Power Band, and it still had EVs after I used the Grepa Berry.
Hoped this helped!