How can I inspect the payload on a UDP forwarding server?

Yes. Change the Virtual Server type to Standard. Since you are performing forwarding, assign no pool, and ensure that "Translate Address" and "Translate Port" are disabled/unchecked.

As you say, you can inspect payload only when operating as a full-proxy. FastL4 disabled that.

Do keep in mind that you can still inspect L3 and L4 attributes for the flow, most particularly the IP addresses and ports, even with a FastL4 profile.