Hi Dave. Please check the announcement at the top of the forum about posting. You seem to be missing something!
You can find the announcement
here(click here). Actually, several people, who are experts on this forum, will most likely not reply if you don't do what is written in that post.
Anyway, once again, since there is no research on the acoustic properties of painted canvas (or at least, none that I'm aware of) there really isn't much anyone can say about that. Painted canvas will reflect some portion of highs, and be invisible to some portion of lows, but the exact effect is not know.
Regarding your treatment: the drum riser you made is a good start, and will be keeping impact noise out of the structure, which is good, as well as probably damping the room a bit, but as you noticed, isn't doing anything for the ceiling. The general rule for acoustics is that you want "hard opposite soft". In other words, with a hard floor such as cement or wood (which is a good ideas acoustically), you want a "soft" ceiling, which means large areas of deep absorption. 4" of mineral wool of about 48 kg/m3 density is also a good start.
that we are going to address by hanging sound gobos and diffusers in that area.
What you describe aren't really "gobos", but rather "clouds". You didn't say how high your ceiling is, but it most likely isn't high enough to be able to use most kinds of diffuser. Diffusers need space to work evenly. Many meters...
So you need clouds up there, not gobos or diffusers. A cloud is just a large frame filled with acoustically absorbent stuff, hung from the ceiling, often at an angle, so you have the right idea with your "paintings filed with mineral wool". The issue is the painting: normally clouds are covered with acoustically transparent cloth of some sort, but painted canvas is not acoustically transparent at all, except at some mid and low frequencies. That may or may not be what you need for your room, but most likely not.
My main goal on this is to diffuse the issues caused by the cement ceiling.
Once again, diffusion on ceilings is seldom a good idea, unless you have really high ceilings. Most types of diffuser (especially QRD types) create lobing patterns around them, that need lots of space to even out and blend together again. There normally isn't enough space between the ceiling and your head for that to happen, except with very high ceilings. Most home studio control rooms are even too small to use diffusion on the walls effectively (but that doesn't stop many not-so-well-informed folks from doing it anyway, just because the saw diffusers in a pro-studio somewhere, and thought that they looked cool...

). So unless your ceiling is at least 4m above your head, I wouldn't be thinking of diffusion up there. On the other hand, since your room is decently large at 45 feet long, you could use diffusion on the end walls, if you wanted to...
Have you actually measured the response of your room, to see where the issues really are? If not, then download REW and do the tests, then post the results here.
That's a nice sized room: you should be able to get it sounding great for drums, if you treat it right.
- Stuart -