Hey, apologies that we still haven’t done this. The Particle System, Trail Renderer and Line Renderer are mostly is “maintenance mode” now, which means they are only receiving bugfixes and very small improvements. This is due to the development of the Visual Effect Graph, which aims to supercede these features.
I did pass this much-requested feature along to the Visual Effect Graph team some time ago, but my understanding is that the first version of trails in that tool won’t support this either.
Sorry that I don’t have any good news on this for you.