That seems like it would be easier than what I mentioned. Maybe that's how they done it. Now, if you and others could do that, all someone would have to do is write a date based mod so that after each patrol, the sub would weather. Most of my patrols start in the month following my last patrol anyway. I assume everyone's does. If not, then by the time you reach home port at the end of your next patrol, your sub will have changed.
The only problem I can see would be the 'dating'. For example: if you started in 09-1939, then your sub would weather each month. But, what if you started in 12-1939? How would you be able to start with a clean sub and have it age from there? Wouldn't you start with a three month old sub?
I can't even begin to wrap my brain around that. That's why you modders and programmers make the big bucks, huh?