In the past I used to call United and ask them to assign a seat for me (for intercontinental flights). They don't have access to seatmaps so they can't see what's available, but they should be able to make a request as see whether it is accepted or reject. However, this wouldn't always work are some of the agents apparently are not aware of how to do it.

The easier option is to call United and get the LH confirmation number, then call Lufthansa and request a seat assignment. For some reason you can't do this online. At 23-hours prior to departure you can OLCI and chose your seats (both intercontinental and intra-Europe). If I don't get the seats I want initially, I usually keep checking the website every few hours after the initial OLCI and you can keep making changes to the seat selection until the departure.