MLPPP Follow Up - How we fixed the problem

FWIW I have also observed that it is necessary to specify the
interface when doing per-packet load balancing across multiple PVCs,

Hmmmm... we're not having this trouble. What are you using to propagate
your loopback interfaces? It works just fine with OSPF.

Probably apples/oranges. We're not talking OSPF with our customer
DSL boxes; the routing entries (as with the original poster, I believe)
are static. Or maybe I am misunderstanding you.