Did Magura ever make a road/drop bar brake lever for their hydro rim brakes, the hs33/hs11 type ones. If not is there a modern lever that would work with them?

I believe they did. Rare as rocking horse poop though, even when they were current.

HS66 and HS77 I think.

HS66 is what you are after(rocking horse poo)
No shifter in it though , although someone (Paul’s maybe?) did do a VERY limited run of converted 8sp ones (rocking unicorn poo)
Good luck finding any!
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Retrobike and American ebay are the most likely places to search, maybe one or two a year come up

Would a modern road brifter work with the callipers?

Not sure.
They are a closed system and take mineral oil if that helps.
Cant think of many modern closed systems.
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There was never a front/left shifter made for them either.

Are road rims up to the clamping force that Maguras can exert?

If he’s using those brakes he’ll have V-brakè mounts on his frame/forks sk probably a cross bike with cross/gravel rims?
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Another thought, Cervelo did some hydraulic road calipers on a P5 TT bike IIRC. Obviously only fit a road frame/wheel but more likely to work with a modern STI shifter?

They are a closed system and take mineral oil if that helps.
Cant think of many modern closed systems.

The difference between open and closed systems is the master cylinder, not the wheel cylinder. I’m pretty sure you could get them to work to some degree with a Shimano lever. Question is whether the master cylinders are of similar bore size.

The Magura rim brake hoses don’t use barbs, so hose compatibility might be an issue with Shimano brifters.

I used to have a set of HS66s. Excellent brakes, and after using them for several years ISTR that I sold them for significantly more than I bought them for. The only time that’s ever happened to me… 🙂

From memory, I think that the hoses for Magura rim brakes are plain plastic extrusions (no reinforcing or over-braiding) so I assume that they operate at much lower pressure than hydraulic disc brakes. It follows that a none matching lever almost certainly won’t displace enough fluid to work them, and if it did somehow push the pads onto the rim there’d be too much force behind them.