There is still no work from Sparx about making the ‘split diagrams’ function available via the public API.
I have had a look at doing it manually (i.e. in eaDocX code) but it’s REALLY complicated: need to understand all the geometry of the whole diagram, and all the shapes and connectors.
So, until Sparx support it, this function will not be available.

Not that it helps you directly, but there’s lots of wisdom out there which suggest that any diagram which is so big that it needs multiple pages is possibly in need of simplification, into several smaller ones, which are easier for readers to understand.