While relaxing after the hard-working period that was the GSoC, I came across a comment left by Torsten Rahn of the marble team. He wondered whether it would be possible to adapt image warping functionality so that it can be used to morph some old maps.
Apparently, they would like to add historical maps to Marble. Unfortunately, these maps are most of the time not "plate carrée projections" of the globe, which is a requirement for Marble. Hence they use xmorph to convert those maps "by hand" into plate carrée projections.