Janome makes one.

Janome 712T treadle machine

It's about $350 online, less than the cost of a battery for solar.

the downside to the Janome is that is does not come with a table. It does not say if it is compatible with a Singer treadle table, but I'd expect that it probably is.

I have several mechanical machines that are treadle convertible, including my current main piecing industrial machine.

Inventions happen when you step away from the crowd. I'm sure it's possible. Is there a market for this?

I donated a number of my old hand crank capable machines to a charity that refurbishes them and sends them to remote locations for people to generate income from sewing. Hand cranks work everywhere, are very easy to set up, and need no infrastructure. I believe solar panels and their associated batteries would be a real issue in remote communities without electrical infrastructure.

I definitely have used a sewing machine on only batteries powered by solar panels when we boondock in our RV. Any sewing machine just sips power, so they have a very low draw. However my RV is set up with big, expensive batteries and expensive panels, charge converters, and monitors. Not the same as a remote location.

It IS possible, as others have pointed out. It's just very very expensive and not practical. And until the price can come down to what most people would find reasonable, there won't be a market for it.

I'm a little confused. Do you mean solar panels that go on your roof and sends energy to your electrical box? or a sewing machine that has solar panels on the machine itself?

I have solar panels on my roof and have not had an electric bill in years. You do still have to pay the electric company for the use of their wires...about $12 a month.

And you would have a battery bank and inverter if using electricity generated by the panels to power things in your home. Solar panels by themselves do not produce the end-usable power. Their output needs to be adjusted to charge the batteries, then the inverter turns the DC power of the batteries into AC power usable by standard household systems.

OP refers to missionary work, so likely there is no utility or existing wire at the location(s) in question.

Solar powered anything is only as reliable as when the sun is shining. And then there's the issue of the electricity produced being able to sustain appliances and machines, Not to mention the high cost And life of the batteries. Treadle machines would be best.