Star Alliance - Seat assignments with travel agent bookings

For the last few years I have made all my own flight bookings online, but now my company has decided to route all flight bookings through a travel agent. Personally I think this is a real PITA but I have to go with the flow. One of my main concerns is regarding pre-assigning seats. I know there will be no problem with UA and LH, as once I have the PNR I can make/change the assignments online, but may be more difficult with the likes of SQ, TG etc. where they may require that I go through the travel agent.

Has anyone experience/advice to offer in regard to Star Alliance airlines, i.e which members allow viewing/changing of seat assignments? This would be for Y and I will be crediting miles to UA

Once you have the PNR of the carrier you are flying on, you can select seats online (unless there's a glitch) with TG, OZ, and SQ. :) You will probably have to call the carrier to get their PNR though sometimes it will show up on UA's itin.