First, we created the branch in the normal way:
svn cp svn+ssh://svn.freebsd.org/base/head svn+ssh://svn.freebsd.org/base/projects/mips
and then started committing to it. So far so good.
Then it came time to merge in changes from head. Here is where we made our first mistake. We merged just the kernel changes, and not the entire tree. The next time we merged the entire tree. The effect of this sequence meant that the changes between the creation and the first merge in the rest of the tree (outside of src/sys) were lost. I don't know if this is the result of a buggy svn client, or if they are a fundamental flaw in svn. So when it came time to collapse the results back into head, we couldn't just do a reverse integration, or even a more conventional merge.
Next, we pulled in pieces of the projects/mips tree into head, either by hand or with the svn merge command. Doing both was a mistake, I think. Although svn coped with later merged from head into projects/mips fairly well, occasionally we'd have issues to sort out.
So, when it came time to merge everything back into head, we were left with a number of difficult choices. We opted to copy the new files/directories in the repo (to preserve the history) and merge patches by hand with svn log entries. The latter went really well. There were no problems with it apart from the odd fuzz and .rej file to sort out (no different than a normal merge). The former, however, causes a lot of problems.
First, it created svn:mergeinfo entries on the files I copied over. This conflicts with the project's "merge everything from sys" edict. This happened because I did the copy in the server rather than the client (which was an attempt to preserve history). It was a simple matter to delete these entries when it was brought to my attention.
Next, somehow we created the files on the branch without the required svn:keywords, svn:eol-style and svn:mime-type properties. This caused problems when we went to commit changes to these files. The precommit complained, at this point, that the files lacked svn:keywords, and we had to add $FreeBSD$ by hand (since the copy didn't complaint that they were lacking). Also, I didn't discover these problems on my own. Other sharp-eyed individuals saw the problems and reported them to me.
Finally, since I did the copies on the server, there was no way to batch them. When I copied a directory, I got the whole directory. When I copied a file, I got the file. Each one generated a commit message.
I'm unsure what other damage was lurking in the projects/mips tree, but the lack of properties, the inability to easily use merge info and the missing commits lead me to the conclusion that it was easier to abandon the branch and create a new branch if needed in the future.
The mips team plans to create more branches, that are more for special purposes, that will be shorter lived in the future. We have no further plans to have one monolithic mips branch that acts as a staging ground for commits into head.