There's a flag you can set electronically, to make the card read only, but if the OS isn't set up to support running from a read-only filesystem, setting the card to read only will make the OS crash. If the OS is set up for a read-only filesystem, it won't try to write to it, regardless of the flag.

Setting up Linux to run from a read only filesystem only takes a handful of commands, but having a tool to automate it would be nice.