Hi Lars, thanks for getting in touch with us. You're correct in that there isn't really a "global" access to get the number, assuming you're using $RevisionNumber - it's only valid in certain contexts of your template. However, I think that this should be accessible at the top level of your filtered report template, outside of other contexts.
I think we can help cut down some of the time it takes to find the highest number, too. By using a where clause and an approximated date range, you could limit what gets repeated, as well as a $ShowOnce to only display the first revision.
If you'd like, please feel free to send us what you've got - email@example.com - and we'll take a look and offer more detailed pointers as applicable. :-)