I think you should open a case with fortinet-TAC. I don' t know where you could begin to manipulated cos settings on a interface and for dhcp. This might be a feature request or start of a feature request if they get enough persons like you that are running into this and using singtel.
Now with that said, I don' t even see how you could get away with this requirement in a cisco router, so before you go purchasing a basic cisco router, you might want to research the hardware requirements.
A cisco switch might be doable, but a cisco router , I would have to say no way can you set the cos value for a tagged interface.
Qs:
Have they given you any sample configurations?
Have you looked at installing a switch in between your fortigate and maybe some how set the cos value? or what does the typical home user deploy when using that service?
Why do they even need this if the two tags are unique?
To me, it seems very restrictive approach with handing out dhcp-assignments.