Cc: @jrevans, @mfitzp as recent contributors to the Qt editor.

I think these are all good ideas. Are you offering to develop pull requests, or just making suggestions?

I need some help to generate QImages with representations of the marker/line styles in order to address my 3rd point. It'll probably also involve fiddling more with fomrlayout...

You can use svg/png for that purpose.

Sure, but I also need to figure out where to put the plot, what size to use and so on. How are the representations in legends created, for example?

The little keys in the legend are produced by the "legend handlers" in legend_handler.py. There is a mapping from each Artist class to a handler class (the mapping itself can be obtained from a Legend object) and the handler class takes care of creating the right artist to draw in the legend.

I'm going to need some more help:

from matplotlib import pyplot as plt
from matplotlib.backends.backend_agg import RendererAgg
from matplotlib.legend import Legend
from matplotlib.lines import Line2D

from PyQt4.QtGui import QImage


p = Legend(plt.gca(), [Line2D([], [], marker="*")], [""]).get_frame()
n = 50; dpi = 72
r = RendererAgg(n, n, dpi)
p.draw(r)
s = r._renderer.tostring_bgra() # endianness, same as backend_qt5agg.py
q = QImage(s, r.width, r.height, QImage.Format_ARGB32)
q.save("/tmp/test.png")

This seems to save an empty image, even though s seems to contain a nontrivial pattern (not just a bunch of \x00\xff\xff\xff but also some other pixels). Any hint of what I'm doing wrong?

Icons is probably too complex to implement to be worth it; the other parts have been implemented already.