make it easy on yourself. wrap it in a shell...
{"name": "Start MYTHING", "priority": 1, "action": "bin/start_MYTHING.sh"}
and then in
bin/start_MYTHING.sh
Code:
#!/bin/sh
/mnt/us/extensions/kterm/bin/kterm -e /mnt/us/extensions/MYTHING/bin/myapp.sh
That's how it is done in the frotz extension.