I have for a long time thought that my computer simply can’t do this, but I recently found and imported someone else’s model that works perfectly for me, so I know I can.
The problem is I cannot get transparency to work in Texture Paint in the Viewport at all. The model I imported did not have any materials at all that I could see, and the texture supplied was a PNG file with transparency, so no Alpha Channel support.
How was this done? If I could actually get PNG transparency on texture files to show as transparent in Texture Paint, without the use of Materials nor Alpha channels, it would be a huge help.