I spoke to a few people about this since commenting, and this is essentially the response I got back — this kind of thing does already exist, but it has typically been organized by ciswomen for transwomen on a community basis — I. E., strong moderation and vetting and familiarity prevents abuse.

So, I'm guessing the better direction for random cis men who want to organize would be to create a chapterized organization that protects or provides physical barriers around places trans people need to enter and may be prevented from doing so by rioters or violent dissidents. My immediate thought is to protect the doctors and patients who give care to trans youth, and just make an email, text, or Fediverse alert system that dispatches people near an area if a credible report appears of that kind of need.

I think, that way, it helps separate communications between trans folks and cis folks for safety reasons, but the needful is still done and everyone comes away feeling better and more united. I feel like that first part is really necessary to prevent opportunistic abuse.