Correct, there is no option to specify where the software will be installed. It will be installed in ${prefix}/var/macports/software.
MacPorts is designed with the assumption that ${prefix} and its private directories therein are real directories, not symlinks. Using symlinks may work in some cases, but in others you may run into issues.
Unix build systems often don't work with paths containing spaces so even though it may be possible to fix this issue, there are likely similar issues with other aspects of MacPorts base itself and with the build systems of most of the ports offered by MacPorts.
If you can submit a pull request that fixes this specific issue it would probably be accepted.
Until then you'll have to rename the drive so that its name doesn't contain spaces.