Very complex rebasing - Need help

So, I grabbed a tarball of the Buildroot project a year ago and started a git repo, and created a branch and did changes:

initial---master
              \
                --o--o--o--o--myproject

A few months later, I grabbed another Buildroot tarball and I diffed it from my master and applied it to master.

initial--o--(new tarball)--master
          \
           --o--o--o--o--myproject

Which I then merged into myproject and did some additional development.

initial--o--(new tarball)--o--master
          \                      \
           --o--o--o--o--o----o--o--o--o--myproject

My problem is that I would like to bring in the svn-into-git repo of the Buildroot project.

initial--ID1--(new tarball)--ID2--master
          \                          \
           --o--o--o--o--o-----o--o--o--o--myproject
other repo of the Buildroot project:
initial--o--o--ID3--o--o--ID4--o--master

So, I can actually match ID1=ID3 and ID2= ID4.

I can use rebase to do this:

In the Buildroot repo:
initial--o--o--ID3--o--o--ID4--o--master
                  \
                   --o--o--o--o--o--o--o--o--myproject

My problem is that I need to kinda-rebase the ID4 into the myproject branch to make it finally look like this:

initial--o--o--ID3--o--o--ID4--o--master
                  \               \
                   --o--o--o--o--o--o--o--o--myproject

Any ideas ?