Ok, first of all, I know this is mostly for SS/FG road bikes, but I thought it would be more appropriate to post this question here than the MTB sub-forum.
I am building up a Monocog Flight. For the time being, I'd like to use the wheelset from my Monocog 29er until I decide on a decent set for the Flight. However, the spacing is off, where it's very close to rubbing on one side. I'm not too familiar with wheel spacing/offset--is this something that I can put spacers on to correct? The actual wheel fits fine into the dropouts. Just wanted it more centered.
First, have you tried adjusting the sliding dropouts? It's easy to get this kind of misalignment when one side (right, in your case) is further back than the other.
If the frame is fine, you might want to re-space the hub (requires cone wrenches) or pull the rim over to the non-drive side (requires a spoke wrench). Go for the first option if the wheel is already pretty true. If it's not, then you might as well true it while you adjust the left-right alignment.
Because both of your frames use the same 135 mm spacing, I would be wary of using a spacer.
My first question is whether the wheel is straight in the dropout. It is difficult to tell from the photo, but it looks like you actually have more space on the drive side at the hub, yet less at the seat stay-- it just looks crooked. I have several mtn bike wheels that I need to "adjust" with the QRs when I swap them. You should have plenty of adjustment, since I assume you have sliding dropouts on this thing. From the photo, it looks like the drive side is further back than the non-drive--- which supports the crooked theory.
Oh and before you mess with the wheel, try installing it backwards and see if that makes it closer to the non-drive side. That would suggest that it's really the wheel that needs fixing. If the wheel still rubs in the same way as before, you're probably looking at a dropout or frame alignment issue instead, as filtersweep suggests.
You say that you tried pushing the dropouts all the way forward. It's probably best to ignore the relative positioning of the left and right dropouts. Instead, see if you can adjust them such that the wheel becomes straight. It doesn't take much of an asymmetry at the dropout to produce a noticeable alignment issue at the rim.