docx4j would need to translate:
Using xml Syntax Highlighting
<w:pict>
<v:line id="Line 2" o:spid="_x0000_s4097" style="position:absolute;z-index:251657728;visibility:visible" from="-3.75pt,.4pt" to="484.35pt,.4pt" o:gfxdata="UEsDBBQA...BQAAAAA=
" strokecolor="navy"/>
</w:pict>
Parsed in 0.001 seconds, using
GeSHi 1.0.8.4
to XSL FO.
http://xmlgraphics.apache.org/fop/fo.html#fo-lines suggests 3 XSL FO structures which may be suitable:
1 Horizontal lines can be drawn using fo:leader.
2 Use a solid border on a suitable fo:block. This will work for horizontal and vertical lines only.
3 Insert a graphic. GIF, PNG SVG, whatever.
See further
http://oreilly.com/catalog/xslfo/chapter/ch06.pdf pp117-119
You should be able to add basic support easily enough using the fo:leader approach, by modifying docx2fo.xslt
Give it a try, and pls contribute a patch if you get it working.