tf_glow is a point entity they recently added to put an outline on a prop. There exists some restrictions though: The glow cannot be seen from other map portals (through most map walls), and the glow cannot be set onto a prop that spawns after the map loads (dynamic entities like a passtime ball).
I've sent in bug reports about these two, but I assume valve has other bigger priorities. If you want though you can also send in a bug report!
You are, at this point, still somewhat better off just to parent objects to flags, or use a
team_train_watcher to 'watch' a specific prop, and then use
tf_gamerules to force the HUD from PL to something else like CP.