You're confusing tags and branches. Tags are not supposed to be modified. They're typically created (from the trunk or from a branch) each time you're doing a release of your applcation.
Branches are used to hold work in progress.
You should checkout the trunk directory to a working copy. To switch to a branch, use the switch command. There's no reason to have separate config files for each branch, sinc eyour working copy, at the same location, can point to whatever you want: thr trunk, a branch, or even a tag. The key point is that the working copy's root should be the root of your project:
So, suppose your project files contains the following files:
index.php
config.txt
some_folder
foobar.php
Your repo would have the following layout:
trunk
index.php
config.txt
some_folder
foobar.php
branches
maintenance_1.0
index.php
config.txt
some_folder
foobar.php
feature_refactor_index_page
index.php
config.txt
some_folder
foobar.php
tags
v1.0
index.php
config.txt
some_folder
foobar.php
And your working copy would be:
MyProject --> references trunk
index.php
config.txt
some_folder
foobar.php
If you want to work on the feature branch, switch to the branch, and your working copy will then be
MyProject --> references feature_refactor_index_page
index.php
config.txt
some_folder
foobar.php
So it would be exacttly the same, and there's no reason for the config file to contain different file paths.