How can I create an exception for a security recommendation for a specific device?

Not sure if this helps at all? but you should be able to most exceptions with device groups i.e

Lets say I go to  - Endpoints - Recommendations

Here it finds 4 servers it recommends doing an update

If I go to Exceptions options I can add a device group and exclude

Even though its a device group that could be used for multiple devices there is nothing stopping you creating a device group with just say 1 machine   So create a device Group here in Settings - Endpoints - Device Groups and then in Filter by Device Group Above just add your Device group to the exception