Sorry I'm a bit late to the party...
Firstly I should state that I use LXC on a fairly regular basis, but have never used LXD. Other than insights shared by others and my brief reading online, I know next to nothing about it. So please excuse me if I say something dumb!
We'd certainly be interested in being involved in some sort of LXD repository. Ideally, we'd probably prefer to just have our images included in someone else's repository (although we'd be happy to host the actual downloadable images on our mirror if that helps).
Alternatively, if it's not too much overhead and could be installed safely and securely on one of our existing public servers, we'd consider hosting our own LXD image repo. However, the actual images would need to reside on our mirror (rather than with the server software).
FWIW we don't currently provide any LXD images. We do provide an LXC template though and a ton of other build types (ISO, OVA, Xen, Docker etc) so adding another supported build type seems like a no-brainer...
We already have a build system! It's called TKLDev. It uses build code (e.g. our LAMP appliance) to build the software appliance from scratch. Unlike Packer it only builds ISOs. However, if you team TKLDev with our BuildTasks scripts, then you can convert an ISO to any of the other build types we support )OVA, LXC, etc).
I know it's a bit sucky to have to learn a new infrastructure to be able to get involved, but once you get your head around it, it's pretty straight forward. It simply uses filesystem overlays and (bash) conf scripts to configure and install everything. Probably the trickiest part is making sure the whole install is unattended and creating the inithooks to set passwords and stuff like that. We aim to be supportive of contributors and open to anyone who wants to contribute.
Having said that, we've been doing a pretty poor job of publishing new community developed appliances lately. As part of the current release (which I'm working on) I'm aiming to remove as much of the stuff that blocks that as possible. One of our aims is to improve our responsiveness to community...
I'll try to keep an eye on this thread in case there is anything else I can add, but please feel free to reach out. Best way to get hold of me is post on the forums; I do spam cleanups most days and try to respond to new posts within a day or 2 (although at really busy times that can grow up to a week). If you have something you'd rather discuss privately, then you can get me at jeremy AT turnkeylinux.org