Hey guys, I've just bumped into this thread and think I can contribute with a simple solution:
1 - In the material, go to the opacity channel and add a Display Color Shader (Surfaces > Display Color).
2 - Then select the object with the material applied, and on the attributes manager, on the basic tab, change the display color to "Custom".
3 - A color picker will appear, this is the property you want to change/animate to control the transparency, just like a black and white mask.
There you go, you can now change and animate the transparency on a "per-object" basis, instead of having to composite afterwards or animate the opacity color of the material itself.
You still have to do this process with every material that you want to be affected (good reminder if you have an object with multiple materials) and you'll loose the material preview in the unrendered viewport (It will be only objects turning from black to white), but it works like a charm.
Here's a project I did using this technique:
https://www.instagram.com/reel/CfrOGgtlxsa/?utm_source=ig_web_copy_link