There isnt. If you want to generate covers on the command line, generate them before hand and pass them to the conversion using --cover
A bit of googling should find the calibre-debug command you can use for generating covers, something like
Code:
calibre-debug -c "from calibre.ebooks.covers import *; d = create_cover('my title', 'my authors', 'my series', 'my series index'); open('/tmp/cover.jpg', 'wb').write(d)"