Reparenting in Mercurial: how does one bring two independent svn clones back together?

Here's the situation: developer Foo created a hg repo from our svn repo. Foo's hg repo was only a shallow clone of the trunk in svn (no svn branches, tags, etc. and the history was incomplete [about 100 changesets]). Developer Bar did the same thing, but cloned the entire svn repo including the entire history, branches, tags, etc. Both Foo and Bar have done branchy development on their repositories.

There is a common SVN ancestor to both repositories, but each hg repo has a different version number for it. I would like to reparent Foo's changes from the common ancestor onto Bar's repo. Here's a diagram of what I'm looking for:

Foo's repo:

C'-D'-E-F---G
       \   /
        H-I

Bar's repo:

...A-B-C-D-J-K---L
            \   /
             M-N

C,C' and D,D' have the same content, but different version numbers & comments.

The goal:

...A-B-C-D--E-F---G
          \  \   /
           \  H-I
            \
             J-K---L
              \   /
               M-N

I've run out of ideas on how to make this happen. I tried convert --splicemap splice.map [splice.map file contained E D] (didn't do anything). Clone -f managed to get everything into one repo, but they appear to be independent trees. After clone -f, I tried rebase --source E --dest D --detach, but it just crashed :(

Ideas?

I'm aware altering the history will invalidate anyone's clone of the repositories, this is not a problem in this case. All users will be re-cloning from the result of this effort.