At last a project that went smoothly for a change. The MAX1238 chips arrived, and I soldered them to the PCBs and they seem to be working just fine at first sight. I was a bit worried about the chips being too small when I saw them, but they are fine.
I wrote a very simple MicroPython library to handle this shield:
class MAX123x: def __init__(self, i2c, address=0x35): self.i2c = i2c self.address = address self.reset() def reset(self): self.i2c.writeto(self.address, b'\x80') def read(self, channel): if not 0 <= channel <= 11: raise ValueError() data = self.i2c.readfrom_mem(self.address, 0x61 | (channel << 1), 2) return ((data & 0x0f) << 8) | data