The discID lookup webservice requires that you provide a discID, even a fake one, before it will attempt a lookup. This should be optional; a plain toc should work.

That is, this works:
http://musicbrainz.org/ws/2/discid/aaaaaaaaaaaaaaaaaaaaaaaaaaa-?toc=1+12+267257+150+22767+41887+58317+72102+91375+104652+115380+132165+143932+159870+174597&cdstubs=no

But this doesn't:
http://musicbrainz.org/ws/2/discid/?toc=1+12+267257+150+22767+41887+58317+72102+91375+104652+115380+132165+143932+159870+174597&cdstubs=no

Nor does this:
http://musicbrainz.org/ws/2/discid?toc=1+12+267257+150+22767+41887+58317+72102+91375+104652+115380+132165+143932+159870+174597&cdstubs=no

At least one of those latter two should work.