I am currently migrating 600+ documents to 600+ spaces in Labs3. I had to reverse engineer the schema with custom properties for the spaces that were created. The documents along with the XML file need to be packaged within the ACP file and uploaded to Alfresco's repository. The upload successfully created the spaces and the documents were properly placed in its respective spaces.
What I did was probably not an elegant solution, however, it got me from point A to point B in a relatively short amount of time. Mind you, I only started learning about Alfresco December of last year. Here's what I did that may work for you:
1. I defined the content model to meet my requirements. Deploy it to your Alfresco instance.
2. Then I manually created the spaces and uploaded the content items as I would expect the import of the ACP file would do.
3. Since the export of the data can be re-imported into Alfresco, I went ahead and exported the various spaces I created to analyze the exported data.
4. After analyzing the data (mainly XML and the folder structure), I was wrote a program to aggregate the documents and created the XML in accordance to the schema.
5. Once you get the ACP created, re-install Alfresco with a clean repository and test and validate the uploaded data.
Hope that helps.