I’d like to create a particle system where I am using a hand drawn texture for each particle. I’d like these particles to automatically face the camera. I’d also like to have their transparency change over their lifetime (fully transparent when first spawned, ease up to opaque + alpha clipping for the middle of their lifespan, and then back to transparent for end of life). Is there a way to do this?
Billboard particles are definitely possible in blender, I did it just six days ago.
Make a plane, set it as your instance object for the particle system, and also, give the plane an object tracking constraint with the camera as the target. I used a damped track, but a locked track should work just fine too. 
As for changing the transparency, you should theoretically be able to do it with the particle info node.
I havent tried using the particle info node in about three years because for me, It never worked, but maybe you’ll have more luck than I did.
Hope this was helpful. Have a good day!
I came across a tutorial for using a tracking constraint, but it has limitations. The thing that is tracked is your source object and all the particles just copy its orientation. So your source object need to be in the middle of your particles and the particles far away from it don’t turn correctly.
As for the particle info node, that might be useful, but I can’t find any way to apply a material to the individual particles. Is there a way to do this?
Thanks.