I guess you've answered the question, without understanding it.
But ok, I'll spell it out... (I thought a wheel guy would get this easy)
You have 3 pieces to the wheel... inner rim, outer rim, and centre/spoked section....
I have a wheel that sticks out too far already so a traditional WHEEL spacer would make it worse...
and I can't machine the mount surface / back of the spoked section down because the spokes already barely clear the brakes...
So, I was wondering if you can use a large diameter ring type spacer in the bit where the 3 pieces meet.... in order to space INWARDS the rim sections, leaving the spoked bit where it is.
I'd just need a spacer ring made up and all the rim joining ring bolts would need to be longer.
I told the guy already it'd be easier to get wheels that fit, but he already paid lots for these from some shark and I was trying to salvage the situation.