I guess you could clean it up with a small Moto-tool grinder wheel and wheel brush and silver braze it. The DO is forged and ground, and the DO is brazed into the stay tubes when the frame was buit. I'd try something like Silvaloy 15 rod. Welding the crack is out of the question as the heat required would probably weaken the DO. Silver brazing is probably safe. If you had a new replacement DO, you could theoretically take the cracked one out of the stays and braze on the new one, but removing the old DO without damaging the frame is a risky and difficult job.