From what I have been able to discern, and a quick test, you can use your floating rule set to the IPsec interface and select your queue like any other interface. The traffic will fall into the respective WAN interface and queue for the VPN connection.
I tested with the following lab setup.
PC1–pfGreenBay-WAN---pfInternert---WAN-pfMilwaukee--PC2
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IPsecTunnel---------------------------------IPsecTunnel
I placed all ICMP in my "VoIP" queue and watched the PPS count on the queues as I ping from PC1 to PC2 and saw the packets show in the VoIP queue.
As for how to "match" the traffic, you can use the DiffServ flags (don't trust them to be there) or by IP/port numbers..... I just re-read you post. I see you are familiar with the DiffServ flags. as for the magic rules? I have no idea I don't work with the wizards much. I don't think I'm telling you anything new at this point but this may help clarify things for other noobs.