They have been used, but it’s typically for extenuating circumstances or hardships that aren’t covered under any other form of leave (IE, your wife is diagnosed with cancer requiring you to be home and you aren’t eligible for FMLA yet, etc).
I highly doubt they’ll allow you a LOA to go make money somewhere else. Explain why you need to quit on good terms (follow the CBA!), go make your cash money, and come back and start over (I’m sure we’ll be hiring). It’s the most fair option for you, the company, and your fellow crew members.