Fortigate 50E - multiple DHCPv6-PD requests with different IA_ID?

[train_wreck]

I currently have a Fortigate 50E firmware 5.6.3 connected to an AT&T residential fiber connection. They have recently rolled out dual-stack IPv6, with the expectation that the customer is to use DHCPv6-PD to obtain prefixes for their devices.

ATT forces me to use a "gateway" device in front of my Fortigate, and according to them, they have allocated a /60 to each gateway. However, the gateway does not allow you to request a /60; you must request multiple /64 subnets by issuing multiple DHCPv6 requests with different IA_ID values. Each different ID will result in a single /64 from the /60. (I will say that I have never come across an ISP that works this way; I almost think that their implementation is broken).

Is there any way to have the Fortigate do this? I have 3 internal LANs, and I'd like to have each with a different /64. I see no options in CLI under "conf system interface" -> "edit WAN" -> "config ipv6" to have multiple DHCPv6-PD requests sent.

[train_wreck]

Hello? Anyone? Does Fortigate support actually read these forums?

[train_wreck]

So, 4 years later, no replies. I still don't know how to request more than 1 IPv6 prefix on the WAN interface..... is this something multiple VDOMs could accomplish? I get it, no one uses IPv6 in the "real world", but still would be nice to know.