Hey there dashiel,
All you should need to do is create a directory called silverstripe-cache inside the webroot (domains/your-domain-name.com/html) and make sure it's writable by the web server user (which it should be by default if I remember correctly for (mt)) and it should pick that directory up instead.
If you want to check what it's doing, do something like this:
echo TEMP_FOLDER; in either your _config.php or after line 57 of sapphire/core/Core.php
HTH.