It's funny how other answers ignore the fact that you can't write to that file...
There are a few workarounds that come to my mind which could help use an arbitrary C:\redirected\settings.xml and use the mvn command as usual happily ever after.
mvn alias
In a Unix shell (or on Cygwin) you can create
alias mvn='mvn --global-settings "C:\redirected\settings.xml"'
so when you're calling mvn blah blah from anywhere the config is "automatically" picked up.
See How to create alias in cmd? if you want this, but don't have a Unix shell.
mvn wrapper
Configure your environment so that mvn is resolved to a wrapper script when typed in the command line:
Note: if you want some projects to behave differently you can just create mvn.bat in the same folder as pom.xml so when you run plain mvn it resolves to the local one.
Use where mvn at any time to check how it is resolved, the first one will be run when you type mvn.
mvn.bat hack
If you have write access to C:\your\path\to\maven\bin\mvn.bat, edit the file and add set MAVEN_CMD_LINE_ARG to the :runm2 part:
@REM Start MAVEN2
:runm2
set MAVEN_CMD_LINE_ARGS=--global-settings "C:\redirected\settings.xml" %MAVEN_CMD_LINE_ARGS%
set CLASSWORLDS_LAUNCHER=...
mvn.sh hack
For completeness, you can change the C:\your\path\to\maven\bin\mvn shell script too by changing the exec "$JAVACMD" command's
${CLASSWORLDS_LAUNCHER} "$@"
part to
${CLASSWORLDS_LAUNCHER} --global-settings "C:\redirected\settings.xml" "$@"
Suggestion/Rant
As a person in IT it's funny that you don't have access to your own home folder, for me this constitutes as incompetence from the company you're working for: this is equivalent of hiring someone to do software development, but not providing even the possibility to use anything other than notepad.exe or Microsoft Word to edit the source files. I'd suggest to contact your help desk or administrator and request write access at least to that particular file so that you can change the path of the local repository.
Disclaimer: None of these are tested for this particular use case, but I successfully used all of them previously for various other software.