There are a few service that allows registration of new web2.0s, like serengines (limited functionality though and limited number of sites), but the closer one to register expired web2.0 would be DHG as you have mentioned earlier - it does not really register, but merely scans whether the sub-domain is available for registration, which isn't really accurate either.
But based on my try, none of these web2.0 would easily release the expired ones except tumblr, and twitter, while the rest of the web2.0 is really looking at a needle in a haystack, and many do not release the old/expired/or disabled sub-domains for their web2.0. Best bet would still be tumblr and there's a tool for this as well.