Welcome to Librarium Online!
Ok so I had an Ulthwe army back in 4th, and was looking at dusting it off but the superiority of Dire Avengers over Guardians (despite the popular thread to the contrary) is making me hesitate. So I was wondering if people think it would be acceptable to run my Black Guardians as "count as" Dire Avengers. I realize this is a weird idea, but even from a colour scheme point Avengers don't fit in with my army where currently the only aspect warriors are Dark Reapers. Anyways just hoping to get a feeling for what people thought.
There is a small chance for ambiguity because they could confuse them for regular guardians who lost their platform or somesuch.
However the old dire avengers were just guardians with slightly different heads. The biggest difference being a large aspect warrior mohawk thing.
Anyway my feelings is that it would be best if you added some piece of funky headgear to them.
Myself I'm thinking of building some old extra guardians I have unbuilt and unpainted as exarcless dire avengers, and I plan to use some extra shoulder thingings of hopefully properly impressive headpieces.
Actually, I would suggest the storm guardian heads. I believe the storm guardians sold now use metal heads different from the plastic guardian heads. Possibly add some other decoration on their armor to make them appear more 'elite' than the basic guardian.
That could set the black guardians out so they could be 'counts as' dire avengers.
There are plenty of people who play "counts as" armies and my ulthwe is one example of that.
I do the exact same thing that you are wanting to do. I don't use normal guardians as I feel they under preform in my army as I am all about speed and hitting one point as hard as I can with everything I can.
I find myself turning to Pathfinders for the 2+ cover and ability to take out harder targets a little better.
So, the only guardians on the table are my "Black Guardians" pretending to be Dire Avengers.
You should have no problem doing this, especially if you don't have any other guardians to confuse people. Even if you do have guardians in addition to your black guardians there should be no problem as they are hopping out of vehicles, are without a warlock, and without weapon platform.
Remember, with the old Craftworld Codex there was NOTHING but the lack of a weapon platform that distinguished Black Guardians from Guardian Defenders. There should be no problem with it today either, so long as you communicate with your opponent.