>For large BGP customers who service many BGP downstreams, the bottom line is >that BCP 38 cannot be reasonably implemented. It's one of the weaknesses in >the system.
Yes, from personal experience BCP 38 should never be implemented buy a transit provider as it will inevitably cause breakage on multi-homed downstream customers for little to no gain and a lot of customer anger. It should be implemented at the customer edge AS, so for a wholesale transit provider is more of a customer education situation. By all means use prefix lists to prevent your customer networks being received anywhere but directly from your customers to prevent them using your capacity without paying for it however.
