I'd think the only way to bring honesty to this is to make exchanges report what their inbound/outbound wallets are, apply the ratios of in/out to reported volu

I'd think the only way to bring honesty to this is to make exchanges report what their inbound/outbound wallets are, apply the ratios of in/out to reported volume gathered from "trustworthy" exchanges, and then you have a fuzzy guesstimate of the volume of these exchanges. Those exchanges that don't submit data are either doing shady stuff, or don't care to be listed (which is perfectly fine).