How does one go about getting your mods up on nexus? Is it an invite-only thing? I guess I'll try to get in touch.

I think you would have to contact the mod nexus owner through the site contact form or IM/Skype contacts listed http://www.nexusmods.com/contact.php, though I understand the general process is just register for the site and then you can upload/download. Without a channel or site for the specific mods though there isn't really a good place to upload them under.

The owner lists his philosophy on setting up new nexus sites here - http://forums.nexusmods.com/index.php?/topic/814062-blog-piece-nexus-development-and-expansion-philosophy/

Which gives a great deal of insight into his philosophy for which games/mod communities he sets up.

ModDB definitely has more games under it's hat, but does still have links to desura on their site, though it's just shown as one of several others on the info pages and isn't called out specifically in their about information. So it's possible it would work.

I know several other of the GOG games are on nexus though, like Witcher 1 & 2, Legend of Greyrock and Neverwinter Nights 1 & 2. So hopefully it would be a good fit for Defender's Quest as well.

In either case, hope this helps!