FWIW, I still own half of Swords of the Middle Kingdom, with Matt Harrop, one of the other designers. We've talked about re-publishing it at various times, I just haven't made any time to seriously consider it.
Heaven & Earth is now being published by
Abstract Nova, who picked it up again after GOO failed. I have no idea what happened to HKAT!---I had tried to get in touch with GOO after the melt-down to see about buying the rights (if they were cheap), but never heard back from them.
Matt may still own the rights to Age of Empire; I don't recall offhand on that one. I imagine that Periphery (both AoE and Periphery were originally published by Epitaph, which EHP later acquired) is still owned by Matt/Gareth/Aaron Rosenberg and whoever else wrote it.
When I've looked, I've seen relatively cheap copies of all of these on eBay or at NobleKnight/etc. Of them, H&E and SMK would be the rarest books, since we only printed 750 of both (perhaps H&E was 1000; I'd have to check). In fact, I still have a some copies of Triad Sourcebook (my only significant contribution to the HKAT! line) up in the attic.