First off, I know this generally not recommended, and a purist would tell me to just suck it up and get a cheap touring frame like a Surly or something like it, BUT

I have an '08 Scott CX bike and I'm wondering if it's a reasonable idea to use it for some shorter self-supported tours. I have a rear Old Man Mountain rack, and would probably be toting 40-50 lbs on it. Will probably use a handlebar bag for some stuff too.

The Scott frame is absurdly light for an aluminum cyclocross frame, but I only weight 135 myself, so I would think that me + gear + rack is still within the acceptable weight limits of the frame. However, I don't know what happens when I put a lot of that weight over the rear wheel.

I've seen people take cross continental trips on Gunnar crosshairs (steel) and the like- would the Scott CX be similarly strong? I've never heard of anyone trying with the Scott... what do you guys think?

I'd worry more about the wheels than the frame. It is worth considering how well a racks going to interact if it's got short chainstays. you might have heel strike issues if you've got large feet.

I have pretty small feet so I'm not too worried about heel strike. I was also planning on buying some dedicated touring wheels...

Really my main concern was loading up the rear triangle... I assume since it's a CX bike that it's built up a little stronger than a road bike would be, but there is so much talk out there about how one shouldn't tour except with a dedicated touring frame, and blah blah blah. I'm not a mechanical engineer, so I really have no way to reason about how much extra force is going into the frame when I put a rear rack on it... and I'd rather not have the bike come apart at the welds mid way through a trip.

...not to mention that the Scott is outrageously light for an aluminum cx frame. I worry about it's durability a little bit.

email scott usa and ask them. my opinion is that racing is harder on a frame than touring. if you were planning on loading it up for a year long journey to la paz via backroads it might be another story but 50lbs attached with a old man mtb rack (it attaches to the axle, I think?) shouldn't destroy it.

(it attaches to the axle, I think?)

Yep. With a special skewer. It also attaches to the brake bosses, though I suspect the axel takes most of the weight.