Out of curiosity I measured a new Avon 100/90/19 'Universal.
The measurement is the tyre/tire sitting naturally and the caliper is set at the inside measurement of a stock 1.85" MB41 rim and what should be the distance add on for a 2.5" rim (give or take)
1.85"
2.5"
I spoke to a local supplier (Long time Commando (and GS it seems) owner and wheel builder) and will look at getting some rims/spokes.
What seems to be overlooked or not posted is that getting 'wide rims and spokes is a spendy proposition even though surprised that the Excel price here was well below that of Buchanan's in the USA (with a post add on)
If you do not build the wheels yourself the cost with new rubber would be close to what a year old Commando could be bought for back in the day so no surprise the number of bikes with wide rims is low in the grand scheme of things.