I found this very useful blog post with a commandline for batch creating icons from an SVG file.
I also want to try to expand it to be able to produce launch screens in the same way by doing an aspect fit to the right shape and produce all those rectangular sizes. I'll post again if I figure it out.
ruby -e '[58,120,29,40,80,76,152,1024].each { |x| `/Applications/Inkscape.app/Contents/Resources/bin/inkscape --export-png icon#{x}.png -w #{x} icon.svg` }'
I'm also considering looking at the Cairo library to develop an OS X tool to do this easily (and possibly also create an XCAssets file). If this app would interest you let me know if you would be sufficiently interested to play $10 for it.
This documentation for Inkscape should give me the necessary parameters to do this with a simple script.
Update - See this Github repo for a fuller implementation
SVGtoIcons is still pretty barebones with no commandline support and hardcoded file names but it should be pretty easy to modify to do what you need. It is only a page and a half of Ruby. I'll write a proper blog post about it soon.