I have uncovered the 'prince-shrink-to-fit' declaration to scale documents that are too wide for the printed media.

I am wondering if it is possible to scale a document that is too long to fit on one page?

No, not at the moment.

Could this be added to the roadmap, if there's no workaround?

It would be very useful to say "scale down this whole element so that it all fits on a single page", particularly for figure elements which contain images and captions that should all be visible together.

You can do this in SVG with the preserveAspectRatio attribute, but not for arbitrary HTML elements at this time. Perhaps this will be possible in the future when we support HTML inside SVG via the foreignContent element.