Lots of the Microsoft Office formats work this way. Change any
.xlsx file extension to
.zip and it'll open right up and let you view the xml markup and other resources. The
.3mf 3D file format also works this way.
So yes, I think you'll be okay to put your own folder inside this zip file and use it to store your documents. However, I am concerned about performance over time... how much work is involved in opening the zip file just to read a record, now that the size is significantly larger to hold all the documents? I might suggest keeping a separate zip file for your documents library. In this way, you're still limited to just two files rather than many, and you're sure not to cause performance issues for normal records.
IIRC, appending a zip archive can be fairly efficient, but changing, removing, or inserting any content always requires re-writing the entire archive. Additionally, extracting individual files used to require reading the entire archive, but does not anymore. So as long as your users only add files and don't change them often, this should be reasonable.