Could the average businessman even draw a crane well enough to distinguish it from any other bird? If not, wouldn't they call it a "broken bird" letter? Even if it's a construction crane – same problem. If they wanted a universally recognizable symbol, wouldn't they choose something simpler? And how could any employer draw a picture of a crane on a letter of recommendation without causing the employee to ask, "What the hell is this?"
And how could a symbol like this spread around so that every employer knew what it meant, but no worker did? I don't think ANY secret code or symbol system could work and remain secret for long.
And if an employee asks you to write a letter of recommendation, and you don't want to recommend him, why not just say "no"? OK, I can think of some reasons why, in some unusual situations, an employer might not want to say no, but in order for a special symbolic language to become well known, a LOT of employers would have to want to use it.
On the other hand, I can believe that such a thing might exist in folklore. In periods of high unemployment, stories might circulate among workers about a conspiracy among employers.
By the way, I tried a Google search, and the only reference I found to "broken crane" as a symbol was in a site about dream interpretation.
I think if you describe a person as a "hard worker" it means he's not suitable for a management position. Your chances would be better if you were described as a "good golfer!"