I always thought the Dawson Rail was a good thing and it was popular there for a while.... too bad it kinda fell off. Your idea to weld the holes up is, I guess, as good as any plan. Probably won't be cheap and I can't say I have a recommendation as to where to send it. Laser welding would be better I'd say and maybe even weld in plugs, but there again, it might cost you more than a new frame. I get it about saving it because it's one of a few, just be prepared for it to hurt a little
I met Dave Dawson in an unexpected place: Venezuela at the Latin IPSC Championship. He was a kinda crazy and fun dude, and later when I saw him called "Awesome Dawson", I knew-- it was true.