It would be great to have some control over automatically correcting widows and orphans.

Maybe a style property that allows you to set the number of px/ems/lines/whatever to extend or contract the main page block in order to keep single lines from appearing at the bottom or top of the page?

There are in fact two such properties, "widows" and "orphans", which each take an integer number of lines with a default of 2.

Please note however that in Prince 5.1 these properties have no effect on content that is broken over multiple columns, they only control widows/orphans in content broken across pages. They will be extended to affect content in columns as well in a future release of Prince.

For the record, Prince now applies the widows and orphans properties to content that is within columns. You can try this out in the alpha version of Prince that we released today.