Today there is no persistent link or reference between a Trunk Project and a Branch from a reporting perspective.
As mentioned you can use lookupfrompath() to report on any table path in Apptio, however I would strongly suggest against this. The main issues that exist with doing this is...
1.)Technically your lookupfrompath() should always point to a calculated build (ABC project_Build_123) When you configure this in your report in trunk it will be a static link, so if someone checks in/causes a new build to spin in your branch, this number will chance and your lookup will fail. If you're build in the branch is never going to change, this is less of an issue, but we still don't guarantee the build number is going to be static.
2.)All !NEWCOLUMNS are evaluated in realtime, we do not cache/persist this part of the path, so the report in Trunk even though pre-calculated may not perform well when using it. How this performs depends on what's being returned, table size, report complexity ect. We do not allow you to use lookupfrompath() in a table transform. I would never use this to back a model object driver.
3.)If you instead point to the Branches base project (Omit the build number) this is something that doesn't actually have a persisted cache, so you'll have all the issues of item 2 above, plus the issue that the source table will also have to calculate on the fly, which will incur additional load on the web nodes.
Almost anything is possible with lookupfrompath() what you build may work and may perform, but if it doesn't we currently don't support that type of reporting/workflow for the reasons I mentioned above, so if there are issues, there really isn't much at this time we can do to remedy those.
If the values in the branch really are static, and not changing often, copying the data from the branch to Trunk is probably a better option depending on your data refresh cadence.