You can download the reference inkscape file, edit the "icon" layer, rename the file accordingly and upload it back. The system applies two filters, one for selected and one for disabled, so that we don't have to create 3 of each kind of icons. You have to resize the inkscape canvas manually to the required size.
I also made a page in the wiki (http://wiki.maemo.org/QtComponents/missing_icons) where everybody interested should help reorder the list so that the most needed icons are at the top, and completely unneeded ones are removed. I will update the list on the webapp once in a while to reflect changes in the wiki.
Question. Is there a free service where one can upload a php script to be collaboratively edited and used? ImageMagick is a requirement.
DISCLAIMER: This little script is very hastily coded, because it is a one-off utility to help us gather most needed icons. If anybody is interested in the code I can share it, but don't expect something nice. Also there is no error-checking whatsoever, the script will probably fail silently if it's fed an svg, not based on the one I provide. Also if you upload something with the same name as something existing, it will overwrite it.
Please give feedback and start drawing some icons. You can have a look at how they are today by looking at your N9 /usr/share/themes/blanco/icons folder or, by downloading the QtSDK and looking into the harmattan sysroot at the same location.