As James has stated, policy acts upon ingress yet you would like it to selectively affect egress. This seemingly insurmountable problem does have a workaround, though I preface the following discussion with the caution that we are now squarely within the realm of "understanding the rules so that they can be creatively broken".
Start with the background in
KB 5888, "Filtering Egress Traffic based on Frame Characteristics" (http://bit.ly/1l5lwNg).
A review of
KB 14443, "Using S/K-Series Policy to identify IPv6 Router Advertisements" (http://bit.ly/IMvR28) might also be helpful.
Let's say that the traffic in question will be ingressing port ge.1.1, and all ports on the system are initially egressing vlan x.
The following variation on the previously suggested policy config would, instead of dropping the IPv6 frames, move them to VLAN x2, which all ports except the LAG should be allowed to egress as well. For this purpose the presence of additional VLAN configurations (VLAN x2 definition, VLAN x2 untagged egress from non-LAG ports), not present here, may be assumed.
code:set policy profile 100 name selectively-BlockIPV6
code:set policy rule 100 ether 0x86dd vlan
<
>
code:set policy port ge.1.1 100
The sequence of events outlined in KB 5888 would take it from there.