Compatibility of 'Unknown' Performance Data Value from #AEN200

Using Icinga and reading as much on the documentation as I could, Service Monitoring Performance Data Metrics appear to completely follow the Nagios implementation of #AEN200 which would make sense for plugin compatibility, but does it accept the literal value ‘U’ for the performance data value? I know that Icinga will not accept string data and there is no mention of the character explicitly in the docs. This would make my network related check far more consistent as network timeouts or other inconsistencies can return a ‘U’ for warning, but in other cases actually return an integer value.
This is most useful when your performance data value is boolean, 0, 1, or optionally ‘Unknown’ due to something outside of your programs control.