Start a Petition
Gamers petition all sorts of companies and groups for changes they would like to see. GoPetition.com’s gaming section is a popular tool for this.
There’s no indication that Wizards monitors online petitions, but they have stated they are keen on input from their gaming community: “D&D has been a resounding success, due in no small part to the feedback and insight we gained from nearly 200,000 playtesters.” (Ref.)
If you got enough support, it is likely you would at least get the attention of the decision makers.
Example: The Inclusivity Petition
A noted example of petitioning Wizards is this Care2 petition which gathered 649 supporters.
How much these few hundred signatures affected thinking at Wizards is unkowable — but that typically the case for any advocacy. However, many people do believe that great progress has been made since the petition.
This is how advocacy generally works: people make their voices heard, and others respond to it. Rarely does an organization point to a particular piece of advocacy and say, “We changed because these people said we should.”
Advocacy comes with no guarantee
I’ve gotten a couple comments asking for proof such a strategy would work. But that is not the question at hand: the OP has asked for a way to advocate for a goal, that is to “to publicly support a particular policy or way of doing things” and make his voice heard.
There is no guarantee that any company will listen to any particular customers advocacy, but they often do, and Wizards of the Coast say they listen to their customers:
We listened and we took notes. We looked at what people wanted from
D&D, how they play the game, and what they value about it the most.
Just as we took the time to get the rules right, we spent time making
sure the core of the game would be delivered to you in the best way
Ever since Sage Advice started in January, we’ve received requests to
gather it in one place. We’ve listened and created the following PDF
to make it easier for you to find answers to your questions
So again, this answer does not guarantee the advocacy will attain any particular goal. It simply provides a time-tested way to advocate for a change. The OP is not asking for guarantees.
Will WOTC see the petition?
Wizards employs Community Managers. Community Management “is about relationships and how your brand seizes opportunities to interact with your community in public online spaces.” (Ref.) A good community manager understands “your community is spread out all over the internet.” (Ref.)
It would be a very poor community manager indeed who did not spot a popular petition to their company making its rounds on the internet — if it is popular, people will be sharing it on their social media. Still, it’s easy enough to tweet the results of a petition to Wizards if you are afraid they might be missing your efforts.