This is related to Bug #207668 In the sourceforge patch tracker there is an IPv6 patch for nzbget version 0.4.0/0.4.1 (see URL). The patch applied cleanly for me and worked precisely as expected. The upstream developer wants to test it on windows before including the patch, but there is confirmation that it works fine under linux. I don't think it would hurt for the ebuild to include this patch, but my feelings won't be hurt if this is WON'T FIXed. At least the info is here for anyone looking for it.
Created attachment 160309 [details, diff] nzbget-0.4.0-ipv6.patch For the sake of convenience, I'll copy the patch from sourceforge's tracker.
I consider this bug fixed with 0.5.1 being in the tree.