AWS ELBs have a series of "policies" which group different HTTPS (read: TLS and SSL) profiles together. It is possible that the "2011-08" policy would be appropriate for this purpose (remains to be verified), otherwise we can define a custom policy that fits our needs. Unfortunately for us, these policies cannot currently be managed in Terraform, so this may end up be trickier than we'd first envisioned.
One possible workaround is to use local-exec to apply the policy manually, as suggested by t0m on IRC: http://paste.scsys.co.uk/488127
provisioner "local-exec" {
command = "aws elb create-load-balancer-policy --region ${var.region} --profile ${var.account} --load-balancer-name ${aws_elb.extelb.name} --policy-name EnableProxyProtocol --policy-type-name ProxyProtocolPolicyType --policy-attributes AttributeName=ProxyProtocol,AttributeValue=Tru