I'd like to point out that to audition for a voice role you send in a resume and a demo. Then, a script is sent to you, you record the audition tape and send it back to the studio. THEN you are hired. There's no face to face in any of that, so unless your resume states that you are trans, the people hiring have no clue whether or not that is the case. So hiring trans people preferentially for trans roles would be difficult as whether or not they know you as trans depends entirely on whether you tell them in advance.
There are a couple really good reasons for excluding information about sexuality/race/etc off an acting resume, namely because it often limits your role potential, an actor's job is not to match the director's image of a character (or in this case a voice), but to plant an image of that actor as that character in their heads. Yes people need to 'fit' a role to a certain degree, but more often than not it is presenting a firm, honest, and well-choice making character rather than trying to read a director's brain. The only exception to this is if you fly in the face of the dialogue (which is not being honest anyway) or fly in the face of the mindset of the character (which an actor is sometimes provided with). In voice work, this is most likely even more prominent as the casting director might not even see your headshot.
And from an actor's standpoint, I think that it always should come down to skills. There's a good reason why so many television shows, plays and the like are up to changing the gender, background and name of a character now a days, it comes to all actors making efforts to make an impression of a character. Any actor can do it, and every actor should in order to get a job. I think preferential casting of any sort is a mistake, as all it does is build resentment, and limit opportunities. One act of preferential casting can and often will lead to others and doing so means that voice acting becomes less about acting and more about who you are outside of it. And that would lead to a lot of great acts being lost in the process.
If we want more trans actors we should start by encouraging the arts in a profound way right out of high school. We need to change our mentality about how a profession in the arts is not an actual profession. The large demographic of white actors has to do with support, something as a brown, queer person, I have never had. Treat the arts seriously, provide more support for struggling actors, and the demographics will get better, otherwise all you're doing is supporting the idea that acting is about physical qualities, which it should never be.
TL;DR Acting requires so much more skills than physical appearance. Real work goes into training both voice and body into an acting machine, and all parts should be open to all actors provided they show the aptitude and dedication to provide a meaningful performance whatever their chosen medium.
Signed,
Struggling Theatre Actor.