Browsing the code Paul linked to above, I saw this note:
// The only known situation when KC_VIRTUALKEY is present but
// vkey is zero is when Alt+Shift is pressed to switch the
// keyboard layout state from latin to national and back.
// It seems that this way the system informs applications about
// layout changes: chm.chr is 0xF1 when the user switches
// to the national layout (i.e. presses Alt + Left Shift)
// and 0xF0 when he switches back (presses Alt + Right Shift).
// We assume this and restore fs, vkey, scancode and chr accordingly.
So maybe Andreas could try hitting the left <alt><shift> before trying to type a euro symbol?
Regards,
EDIT: looks like Feathernotes uses <alt><shift> to left and right justify text, so probably not gonna work unfortunately...