The tailwheel lock is part of the default gauge, I would have no idea how to add any function to a gauge. My only edits to this gauge are graphic changes. FWIW, I tried the default DC-3 throttle (with the "battleship" background) and got the same results - it appears normally if placed on the main panel but not on the popup.
[Window01]
file_1024=enginepanel.bmp
size_mm=387,445
window_size_ratio=1.000
position=8
visible=0
ident=THROTTLE_PANEL
window_size= 0.190, 0.350
window_pos= 0.100, 0.650
gauge00=DC3Gaugesnb!ecu, -1,-38,387,406
gauge01=Douglas_DC3!pitch_trim, 88,61,8,143
gauge02=Douglas_DC3!rudder_trim, 124,288,63,59
gauge03=Douglas_DC3!aileron_trim, 209,288,63,59
gauge04=propktmags2, 117,369,159,73
gauge05=MAPE_Halifax_Mags!Starter1, 61,380,42,42
gauge06=MAPE_Halifax_Mags!Starter2, 293,380,42,42
gauge07=cpt.toggles.black!Fuel1, 20,385,26,31
gauge08=cpt.toggles.black!Fuel2, 345,385,26,31
gauges 00-03 are all part of the throttle (ecu) gauge; the "nb" is one of my edited gau files - the default gauges with no bezels, delete that, so it is DC3Gauges!ecu, and it will use the default gauge.