Classic! When I worked in bike shops, people would ever so often bring in a rear hub with freewheel still attached. It was sad having to explain to them what they'd done.
One fix is to make some Z-bend spokes, which don't have heads on them, so they can be inserted into the flange. They can be made by snipping just the top cap off the J-bends, then adding another bend near the existing bend, with narrow-nose pliers. The space between the two bends should be set to match the flange thickness of the hub, but it doesn't need to be precise.
You can make, say 8 spokes in the time it took me to type this.
The exact length of the spokes also doesn't matter. I said 8, which is based on one-fourth of a 32-hole rim, but the exact number doesn't matter either.
Once you have the spokes made, lace a wheel only to the right flange and only pulling spokes, no crosses. Thread the nipples on to full engagement, and proceed to removing the freewheel the normal way. Once your 8 (or however many) spokes are pulled tight by the freewheel removal action, you'll have almost the same removing power as a fully built wheel, no crossing spokes needed.
It is possible to damage a rim by this method so you might want to choose a junk one or at least a heavy/strong one. You can't hurt the spokes though, you aren't strong enough.
Remember to hold the freewheel remover tightly to the freewheel with a QR skewer. Back off the tension on the skewer after the freewheel breaks loose. (Standard advice for almost any FW removal.)
If we charged a customer what this job should cost in normal labor rates, most people would opt to throw the hub and freewheel away. That's always an option; used vintage hubs and freewheels aren't expensive, like on ebay, leboncoin or swap meets for example. Extra karma points for keeping these out of the recycle bin though. Good luck!