I was able (eventually) to repair the holes using a product from Caswell called
"Muggy Weld Super Alloy 1".
It has a melting temperature of 350F, which is a lot lower than the pot metal, and you can tell when you're getting to the right temp because the flux turns brown. The solder doesn't turn completely liquid all at once, but first gets ... "sludgy", which allows you to fill in holes.
I actually made the repair about 5 times, because after filling in the holes, I used a Dremel tool to grind the resultant globby mess into shape. Invariably, as I was getting to the point where I thought it would be good, one or more of the holes would reappear.
So finally, I left the bottom of the carb slightly egg shaped, to leave a thicker layer over the area with the holes.