Got it:
if event.propertyName in ('componentRunning'):
from java.util import HashMap
from java.awt.font import TextAttribute
fontAttributes = HashMap()
fontAttributes.put(TextAttribute.UNDERLINE, TextAttribute.UNDERLINE_ON)
obj = event.source
obj.setFont(obj.getFont().deriveFont(fontAttributes))
Edit: worth noting that to turn it off, there’s no corresponding “UNDERLINE_OFF”. Simply set it to -1.