I kind of have it working using this:
KNS81_chkButton_Press#1 (>L:var_RNAV_CHECKMODE, bool)
KNS81_chkButton_Release#0 (>L:var_RNAV_CHECKMODE, bool)
When you press and hold the CHK button it will flash the DME info and flash back, then flash back to DME and remain displaying it with an occasional flash, then revert to normal display on release. Everything else is working perfectly and I am very happy with Blacksquare for doing both this and the KNS80 RNAV units. My father had one of the very early KN74's installed in his Navajo Chieftain in the early 70's and loved using it. I remember watching the drum numbers rolling to their settings.