From http://docs.info.apple.com/article.html?artnum=9502

* You can still create an MFS volume on a single-sided disk by holding down the Option key (and keeping it held down until the formatting is complete) when selecting Erase Disk under some versions of System 7, including System 7.0x and System 7.1. However, you cannot create an MFS disk under System 7.5.

So, if you aren't running anything newer than 7.1, you might be able to pull it off.