It is not the case here, as he connects it to battery itself, (not USB) when need arises.Which, of course, mean even more risk in case of over-voltage - battery circuicity should just cut it off, but you will still have overvolt on N900's battery terminals, which may fry everything (even more, due to 0 load from battery, when it cuts-off in emergency mode).

Of course, there is risk only, if that panel is ever capable of providing more than 4.25V (even for s hort while). Everything below should be safe.

OP said, that normally, this panel isn't connected to battery (isolated via tape), and he mean to use it only in emergency situations. Even then, over-voltage could be catastrophical, so I would check that panel's output in different conditions, extensively.

/Estel