I have a feeling that it has been found unsportsmanlike. However I think its probably rare for there to be a start sequence at which there are no incidents that one could raise a half credible protest for.  Does rather emphasise what a nonsense the rule is though.

Anthony let's look at the rule.   (Emphasis added)

The sentence structure is "she shall [action] ... at the first reasonable opportunity for each [incident]"

The "at ... for each" required the [action] to be done at a time that is associated with the "incident observed". 

I don't think raising the flag at the start breaks a rule  .. but IMO doing so would make any subsequent protest invalid, due to the conclusion that the act of displaying the flag was not displayed "at ... for each incident". 

If however .. there was a protest at the start, this boat is required to continue flying it for the duration of the race and subsequent protests only require a hail. For multiple incidents, the flag is displayed at the first incident. 

If there is subterfuge involved and uncovered, then they do risk those consequences.  
 
The juice doesn't seem worth the squeeze.   Just figure out a handy way to keep the flag ready to fly. 

This would be one of those cases where, of you have an a*hole racer that does this, maybe it's an RSS violation, but you simply add a sailing instruction that forbids the practice. Or take them aside and tell them to knock it off.  If you are the sailor contemplating doing this, you may wish to consider the meaning of "sportsmanship."

Caveat: English is not my native language

Angelo, for me "each" referes not to incidents but to the two actions "hail" and "display red flag". So a boat is obliged to do each at the respective first reasonable opportunity. 

For what it's worth I asked Google's opinion on that matter. I think it kind of backed my understanding (emphasis by Google):

"In this sentence, the word "each" refers to the two required actions: hailing "Protest" and conspicuously displaying a red flag.

Grammatical Breakdown

The phrasing is taken directly from Rule 61.1(a) [sic!] of the World Sailing Racing Rules of Sailing. The phrase "at the first reasonable opportunity for each" means:
- The boat must hail "Protest" at the first reasonable opportunity for hailing.

- The boat must display the red flag at the first reasonable opportunity for displaying the flag.

The rule recognizes that a crew might be able to do one action before the other. For example, a solo sailor can instantly shout "Protest" during a collision, but they may need to untangle lines or stabilize the boat for a few moments before they can physically reach and hoist the red flag. Under this rule, the timeline for evaluating a "reasonable opportunity" is judged independently for each action.

The heading of rule 60.2 is "intention to protest". So a red flag signals your intention to protest. I'm not sure what it means, if it's displayed but ther is not actual "intention" (yet?). Also what does it mean when you cross the finishing line with the red flag but no grounds for a protest?