Why not simply use configuration option Cisco gives
you to set your DS3 to 6 meg dsu bandwidth X
That's what I do, works fine.
Dan, your suggestion will unncessarily tax his equipment.
Not only that, but the rate-limiting on the input interface will likely force the customer to do some QOS at their end too; the discrepancy between what the customer's router thinks the bandwidth is and what it really is will cause packet loss. I like the solution of the sending interface to queue the egress traffic at whatever speed is available better.