Well, I put one of those damn S5Timers in my UDT and that worked. When I tried to use it in my FC, S7 gave me a "Compiler Information: type confict." error.
I'm attaching a picture of what the setup looks like. I declare my UDT, being passed into the FC, as an IN/OUT parameter. As you can see in network 23, I have no problem tunneling down into the UDT and assigning paramter addresses. But it won't accept the address for the S5Timer in network 22, which is clearly available and visible in the declaration area.
(EDIT: Okay, I can't add the picture, but I'll keep trying.)