You CAN select seats prior to check in, and there are several ways to do it. One requires the record locator number (some may require the PNR number instead) while the other does not.
With Record Locator:
Some airlines will not allow you to see the flight information even if you have a record locator number (looking at you American Airlines) when booked with a third party consolidator like FbC, but there are several hacks by using partner airlines websites to do so.
We fly exclusively One World partner airlines so once I have the record locator, I head on over to the Royal Jordanian Airlines website and insert my records locator number in the Manage My Booking page. From there, I can see the seat map (also availability) and our assigned seats for each flight. If the seats automatically assigned are acceptable, I move on with my life. If not, I jot down the seats I would like and then call FbC with my preferred seat selection. They usually make the changes while I am still on the phone with them.
Without Record Locator:
I use Expert Flyer to check each's flight seat map and availability. With my preferred seats in hand, I call FbC and have them assign those seats to us and then ask them for the record locator. I then verify that the seats were correctly assigned to us by using said record locator on the airline's website (or partner site).
It's been awhile since I have used FbC, so some of the above methods may have changed. I know there are quite a few fellow CC'rs that use hacks like I have described above, so I am sure they'll chime in with any updated information.
Hope that helps.