I don't know if there may be other (better? easier?) ways to do this but you can certainly add an "onKeyEvent" function in your GridScreen.brs something like this:
Function OnKeyEvent(key, press) as Boolean
result = false
if press then
if key = "up" and m.button.hasFocus() = false
m.button.setfocus(true)
result = true
else if key = "down" and m.button.hasFocus() = true
m.rowlist.setfocus(true)
result = true
end if
end if
return result
End Function
(Note: this assumes you've done a findNode on your button to set m.button)
aspiring