flagger indicator

If I'm already going for health when the flagger shows up, I might not react quickly enough to let the flagger have it.  To them, it might look like I'm stealing the health.

A much simpler suggestion is in my second post.

Suggestion: An indicator of when the ally flagger is approaching and what direction they're coming from.  Maybe a visual indicator for direction plus a sound alarm.  You could design it to apply to only the players within certain map regions.
The equation could be something like proximity ÷ average speed.
So if we wanted a 0.75 second heads up, the code could say something like
When (x-axis movement over the last 2 seconds × 0.375) is less than (flagger's proximity + ½ screen-width), sound the alarm.
If the flagger is above or below, i.e. when the y-axis distance is greater than x-axis distance, you could replace x-axis with y-axis and screen-width with screen-height.  You could also go with pythagorean theorem, but it would have to be doing this computation constantly for each player.

Alternative:
Sensors placed and labelled by the map creator.  When a flagger passes them, teammates get a message like "2B" or "flagger at 2B".
This would probably be lighter on the servers because it wont be performing a distinct calculation for multiple players.  However, a potential problem is that a traitor could start feeding this information to the other team until they are finally kicked for it.  If an opponent can already see the flagger when this begins, they will know whether they're being fed truth or lies.