I've tested a bit that code.
And unfortunately, Turbine doesnt want us to see some1 target (even if in party/raid).
The TargetChange fires ONLY when you have selected the member of your party.
Once you unselect him - it doesn't fire anymore on this member, which is really sad =\
I've tried both solutions, adding a callback and assigning function for TargetChanged event directly. But they both work the same way.
I believe only way to make it work - is SOMEHOW, from LUA script make/simulate Lotro think you have selected the target player (actually all of them, in raid or party, to handle all members' targets), then yeah, callbacks gonna work. Btw, do you think that possible to do somehow ?