You could smuggle it in using Soil temp or leaf wetness via the PWS upload process:
http://help.wunderground.com/knowledgebase/articles/865575-pws-upload-protocolsoiltempf - [F soil temperature]
* for sensors 2,3,4 use soiltemp2f, soiltemp3f, and soiltemp4f
soilmoisture - [%]
* for sensors 2,3,4 use soilmoisture2, soilmoisture3, and soilmoisture4
leafwetness - [%]
+ for sensor 2 use leafwetness2
The alternative I can think of is to write a little macro (AutoIt or *nix similar script) to change the status. But the former methods should show it a bit easier?