Its a little more complicated.
Anything in V and under (LH fare class) is only allowed advance seat assignments for a fee, kind of like AC does for Tango fares. So theoretically, if booking on LH in a lower fare class, you should be able to call in and pay $X for an advance seat assignment.
However, the complicating factor is the codeshare - if the ticket is on non-LH stock, LH apparently can't do an advance seat assignment at all on these lower fares, as they can not allocate the cost to a ticket that is not theirs.
There's a big thread in the UA forum on this issue. So not limited to AC codeshares. I could be wrong, but I think there might be a similar issue for someone who buys a ticket on UA-codeshare, AC-operated flight when it books into Tango.