Most of them have been coded to be opened via script, or by changing a setting in the config. Unfortunately those that haven't may still need to be coded internally to open. If you find one that opens in single-player that isn't enabled in LU, be sure to post a suggestion for it to be supported too!
As for the solid ones, many of them are a full object that is part of the building itself, and cannot be opened. You can probably test this by spawning the building's object ID, and see if the door is attached to it.