...They could probably reattach a nacelle that shook off when nobody listened to Scotty's "Me poor bairns!", or a saucer half that got broken off by that Giant Green Hand, yes. Whether this would be related to the saucer having been designed to separate or reattach is debatable.
We never heard of a starship that couldn't separate, or couldn't land, or couldn't reattach, or couldn't take off, or, say, couldn't fly through water or mountains. There's mild wow value to the E-D reattaching for the first time, but nothing definite about how unique this maneuver really is. OTOH, there's plenty of lack of mention of saucer separation in TOS or DSC adventures that involve trouble with that other section of the ship. Might be it can't be done. Might be it can be done but is unwise in that specific situation. Might be it is only wise if reattaching then follows, and that can't be done.
It's pretty odd to have a saucer in the first place if it can't separate, though. Are these odd dangling bits and thin pylons used solely so that the dockyards have it easier swapping the components? Or is there a field application involved?
Timo Saloniemi