I don't immediately see a solution, but it would be easy enough to script something up. What I give you is not bug tested.
First you would make a list of directories that you would want exclueded. Called ~/.svn-merge-excludes
#regexes to exclude
./helpfiles/
./deploy_scripts/
./models/customer_integration
Then you would write another script to revert those directories in the branch after you merge the turnk into them.
#/usr/bin/bash
# I am assuming that you made the excludes based on the root directory
# so that relative paths will work
for dir in $(cat ~/.svn-merge-excludes) ; do
svn revert -R $dir
done
That should work. The big thing here is the '-R' option in the revert acting recursively on the directory. Therefore your workflow will look something like.
# cd path/to/branch
# svn merge -r123:245 svn://svnserver.com/svn/trunk
# revert_excludes_from_branch.sh
Then you would handle the conflicts and then commit. If you are using windows then a batch file would work too, you could just type out the revert commands in series.