I've just uploaded a tcx file, but the power does not show up in Endomondo.
See example:
<Trackpoint>
<Time>2014-07-19T08:29:52Z</Time>
<AltitudeMeters>10</AltitudeMeters>
<DistanceMeters>14.027777777777789</DistanceMeters>
<HeartRateBpm>
<Value>85</Value>
</HeartRateBpm>
<Cadence>40</Cadence>
<Extensions>
<TPX xmlns="www.garmin.com/.../v2">
<Watts xmlns="">100</Watts>
</TPX>
</Extensions>
</Trackpoint>
It works in Strava though.