I finally figured out how to use synfig to create titles for inclusing in a video with non-square pixels, such as PAL DV or similar.
Use synfig as normally, then when rendering, check out this page on synfig.org, note 6. This will allow you to render a number of pngs.
You can now use kdenlive to import these, using the slideshow command and overlay them on something else. If this does not succed for you, you can export your clip to pngs (perhaps kino is easier to use for this, or mlt or ffmpeg). You can then overlay your background and foreground clips, using
composite -compose src-over $FOREGROUND $BACKGROUND -depth 8 $OUTPUT
Note that the -depth 8 is important if you wish to use them with kdenlive and ffmpeg, as composite will use 16 bits by default, and this does not appear to work with kdenlive/ffmpeg.
Finally, if the slideshow import fails for you, you can create a clip using ffmpeg, something like this:
ffmpeg -i Output/output%03d.png -aspect 16:9 file-169.dv
where you replace your aspect and %03d as needed.
If you are interessted in titles, make sure to also check out titles for OpenMovie with inkscape. Note, that synfig should be able to import svg’s from inkscape, so you can create in inkscape, then animate in synfig.