Sorry to have to disappoint you: Aperture 3.3.1 still does not sync libraries.
But, if I understand you correctly, in your case the "merge" should suffice; it will work, when you only change one library:
- When you are done with editing on your MacBook, export the changed projects as a new Library: File > Export > Project/Folder as Library.
- Import this library into your main library; use the "merge" option, not the "add" option.
- When prompted how to resolve conflicts, select the imported library; this way the edited images from the MacBook will replace the older versions on your main Mac.
That is what I use when I am travelling with my MBP and need to get the changes back into the main library on the iMac when I return.
The prompt when importing the library "Leuchttuerme"
and the prompt how to resolve conflicts:
One tiny addition which, if I understand correctly, will exactly fit the OP's desired workflow. No need to export from the "mini-Library". From the main Library, create the mini-Library by exporting just the Projects which contain the Images to be worked on (since you are going to be making adjustments as well as changing metadata, make sure to check "Copy Originals into exported Library"); then after work on the the Images in the mini-Library is done, import & merge the entire mini-Library into the main Library. (And then delete the mini-Library on laptop, if a copy was left there.)
OP: I'd make the faster of the two machines my base for Aperture.