As far as turning back to a 10 speed, if you want to do that and and some "quirkiness" , how about make right shifter front derailleur and left rear?

Alright, I have ideas. But before I can spill the beans, I’ve a few questions about your spec and design constraints:

1– are you amenable to more cable stops being clamped onto that lovely paint job?
2– must the bell stay on the stem?
3– is that front brake nutted or recessed mount? (looks recessed but want to confirm)
4– how much clearance is available between tyre and fork crown?

Use a 3D printer to make a shift lever replica which just slips over the existing bell ringer and latches on.

It will poke up more than the one you have and will be easier to actuate.

You don't have to change anything and the bell will ring as well as ever. If it gets lost, stolen or breaks, print another.

Why half step?

Because sometimes it's an overstep.

Not a bad idea, but I already paid 50 bucks for the nostalgia.

Thinking of handicapped painters who have headgear to hold the brush, why not attach a drumstick to your helmet? The Nod-bell (patent, not pending even a little).

boy, when I was still working, I bike commuted and dealt with a few rude motorists (but have to give credit to many folks who were very considerate and respectful!)
In my work in the electronics part of the Big Yellow Machine company, one of the folks handled all of the horns, alarms, etc. that were used on the machines.
He had some air horns on his desk, and I was quite tempted to ask for some used parts for my bike!
The thought of getting buzzed by some jerk and then hitting the button of a big air horn was quite enticing!
However, once I started running the numbers for the size of the battery required to power the horn, I decided that it wasn't really worth it.

If someone does want to play around with an amplifier and speaker, there are mp3 files of train horns available on the web.

Steve in Peoria

I've wanted a small bazooka, perhaps inside the top tube, with a heat seeking rocket that would fly up the offending tailpipe, whereupon a very heat resistant balloon would inflate, stalling the car. In about 10-15 minutes, it would deflate, car would start normally and driver would drive off clueless.

Of course, this doesn't answer the OP's question. I think the shifters look just right as is on the DT so I'd go a different route. Maybe a silver spoon (or would lowly SS sound better?) I'd want the pivot on the stem under 6 o'clock on the bell. Striking at 10 o'clock. So you brush your hand back beside the top of the stem to pull back the lever to strike.

I was buying some bike stuff recently on CL from a fellow whose dad was a cyclist and had passed. While I was there I picked up one of these for $10. I don't think I'll actually use it but it's an entertaining idea, and LOUD.

Have one of these and it is loud enough to get car drivers attention.
Amazon.com : Hornit dB140 Bike Horn (dB140 V3) : Sports & Outdoors

In all seriousness, shifter bosses are threaded M5. If you can find a bell that has an M5 thread, just screw it in. If the bell is female threaded, a bit of M5 threaded rod will work to mate them. Or hack off the head of an M5 bolt, clean the threads, and use that.

If you have a bell that you'd like to use and it doesn't have an M5 thread, the Atelire could silver braze on a bit for you. I've done it before for another application. If you want or need a vintage bell, I know a guy.