This is a really tricky one, since Sparx don’t make available any APIs for manipulating images: we just get to copy it to the clipboard, then paste into the document.
So, the solution is, sadly, partioning the diagrams in smaller bits… 🙁

I’ll investigate, but I worry about slowing-down the generation process.

We have now had a look at this, as we have had the same request from other customers.
Our first approach is to encourage Sparx to give us access to the code they have already written for their RTF generator. Another of our customers has already placed a request in the Sparx ‘Request a Feature’ section, but if you can please submit one as well, that will realy help – Sparx seem to deal with these on a ‘most requested feature’ basis.
We have also spoken to Sparx separately, to provide additional encouragement.