I have done that, the problem seems to be that the build process tries to pull files from $LB*MIRROR* wheezy-updates and fails.
When it tries to get files from $LB*MIRROR wheezy/updates then things work better.
I have not worked out where the wheezy-update and wheezy/update comes from. I found some in preseed.cfg, but changing them didn't appear to help.
(And I even tried changing those to distinct nonsense values to see if they were relevant, and it appears not at that stage)
live-boot | grep mirror
gives you some cupe of options.. also can be build from jessie install using distributions as "wheeze" in parameters of live-build
If I can get Jessie running on a 3.16 RTAI kernel then a full Jessie ISO might be a workable solution for the next year or so.