I'm sure the airplane itself will fly happily at higher levels, as the 25000ft limitation comes having oxygen for only 10% of the passengers, so legally it has to reach 10000ft in an emergency descent in 4 minutes I believe, not more.
The Vmo being 250kts, this gives a max altitude of 25000ft.
But the real issue in flying higher than certified comes from the fact, I think, that the cabin altitude will rise above the limit and trigger the master warning.
Yes you can silence it, and accept a steady red caption on the CCAS.
No big deal if you are a non smoker
But in doing so, there will be nothing to protect you from a real slow depressurization, besides from keeping an eye at all times on the cabin altitude indicator.
Thus the risk of fainting, and..... hasta luego!
Btw, another ex colleague of mine did the same thing from Europe to a Middle Eastern Sultanate...
Unfortunately for him, that didn't go unnnoticed, and he was grounded with no salary for six months
