I would guess (never having been in any of these special issues) that you submit to the 'main' subscription magazine from which the special issue derives. Or maybe they put out calls for submissions...or perhaps all the projects are commissionned specially... can any of the Interweave folks enlighten us here? How do you fill your special issues?
My website: http://lynndavybeadwork.co.uk/
We've just finished Creative Jewelry which is on newsstands now. This is a special issue publication that comes out once a year. We solicited projects for the magazine from artists we know and also received inquiries from new artists that were familiar with the magazine. In addition, we selected contributors to Step by Step Beads that had pieces we felt would be better suited for Creative Jewelry. If you have something you would like us to consider, please send a jpeg to firstname.lastname@example.org and we'll be happy to take a look!
Editor, Creative Jewelry
Managing Editor, Step by Step Beads