I know this feature is ongoing, but with the 2.0.0-RC builds the saveIncremental (without signature) stop working at all. A ByteArrayOutputStream is used in the COSWriter for output. This OutputStream will only be handled in the case, when we write a signature. Otherwise the whole content will be discarded.
As I wrote some time ago on the mailinglist, incremental update work in a limited way. At the moment we use it for augmenting signatures and this works with the old 1.8.x but not with trunk after the patch
PDFBOX-1847 was applied.