So basically, we need someone to hire a hot woman to sleep with a Panasonic FW engineer, get him drunk, and come back to us with a copy of the FW so someone can make some modifications.
Seriously though, I'd be careful getting the voltages adjusted even if it's possible, as they may have done that based on what voltages are the most stable during set periods during a plasma's life (and also to avoid things like IR/BI). Without an official word from someone at Panasonic, it'd be hard to know exactly why they they designed it that way.
There's basically three options:
1) they were unaware of it and are too lazy to fix it
2) they intentionally engineered their plasmas to get worse over time to make people want to upgrade
3) they intentionally engineered their plasmas to operate as best as possible over the course of their life, taking into account the changing phosphors and electric tendencies therein.
So if 1 or 2 are the case, you're fine - but if it's 3, you could potentially fry your set by providing voltage outside the acceptable range for that period of its life. This is speculation though, so maybe someone more knowledgable than I can comment on it.