Your LCD probably doesn't use EEPROM for storing special characters but dynamic (or static) RAM. This does not have the kind of WRITE limitations that older EEPROM designs had. After all, your PC's RAM is updated thousands of times a second, if it had that kind of limitation your PC wouldn't last more than a minute!

How can you check whether you have EEPROM or dynamic RAM? Create a custom character, save it to LCD, power off, and check if it's still there a couple of hours later! If it's gone, then you're safe. If it's still there... well... you still might be OK...

Technology is in a state of continual flux, you have some new PICs on the market today that have many kB of static RAM (the kind that doesn't get erased when you power-off) and which doesn't have the EEPROM life-limit. There's also EEPROM's on the market that last millions of WRITE cycles - not just a 100k.

So, in summary, what I've written probably won't help you. Your LCD's Datasheet and/or manufacturers technical helpline is a better bet.