URPF on small BGP-enabled customers?

Am I missing something obvious here?

Try announcing the remainder of yopur prefixes with either a couple
prepends, or see if SprintLink supports the use of communities to
control advertisement (you want SprintLink to prefer the paths they
learn from you in case it's strict uRPF, but not pass those
announcements along to preserve your current inbound policy as much as

That will (in theory) let you advertise all your prefixes to
SprintLink with minimal change to traffic flow, and if your
dropped-traffic problem goes away then you'll have a good supporting
argument in favor of the explanation that uRPF is in effect.