and there really aren't any docs about it (including trunk docs)
There are no docs about it... other than the Free Pascal Reference Guide (of course... for you, that's not documentation.)
If I refer to documentation it is always the true documentation, but there is no reference to bitpacked objects in the reference guide.
That does not mean that would not exist.....
I almost monthly report on omissions, unclear or wrong fragments of documentation. This is usually very quickly repaired or explained.
"There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy." read: documentation can cover...
- Hamlet (1.5.167-8), Hamlet to Horatio