I’m pretty sure the answer is “yes,” but can you use one/some/all of the attacks granted by Multi-Attack Specialist to perform a multi-targeting attack? Obviously the disadvantage would be fairly brutal, but I think this would be an interesting character build.
CAAAAAAAAAAAAAAAAAAARL!
(I’m so, so sorry. I just had to 
)
“Example - Vax the Deathbringer has Multi-Attack Specialist (Tier III). He declares that he will be making three attacks this round (two additional attacks). Therefore, all of his attacks suffer disadvantage 3 (3 x 2 = 6, minus 3 for feat tier 3). His first attack is a necromantic burst targeting a 10’-cube. Because of the area of effect, Vax suffers an additional disadvantage 2, making his total disadvantage for that attack 5. For his second attack, Vax casts a spell of blindness on a single foe, making an action roll at disadvantage 3. For his final attack, Vax moves in to melee and uses his touch of death on three foes. Targeting 3 foes incurs an additional disadvantage 3, making his final attack roll suffer a total of disadvantage 6.”
http://www.openlegendrpg.com/feats#multi-attack_specialist
TL;DR Yes, you can.
Funny how I missed that in the example. Thanks!