Mark -
You are correct. It appears that the current release of the spectrumgtw probe does not allow you to filter alarms passed on to Spectrum based on the value of the visible field of the alarm. That would be considered an enhancement request for the spectrumgtw probe. There is currently no option available out of the box that will allow you to filter UIM alarms to prevent synchronizing invisible alarms with Spectrum.
You might be able to create a nas Auto Operator profile that adds a keyword to the message of invisible alarms. In that case, you would then be able to create a filter on the message field of UIM alarms and exclude any alarm where the message contains the keyword from being synchronized with Spectrum.