I can't help but think that the bulb filaments are failing because either:
a) They are burning out due to an overvoltage situation.
- or -
b) An intermittent connection is [thermally] stressing them.

For a) I'm still unconvinced that we've been able to rule out an intermittent voltage regulator at the alternator. I'd imagine the increased brightness of the lamps would probably go unnoticed while driving on busy roads with streetlighting. The dashboard lights might give a clue but OTOH, they are quite likely to be PWM driven and therefore less susceptible to voltage fluctuations.

Have you had an OBD [etc] diagnostic reading taken? Taken from here, there are a number of helpful fault codes that the ECU might have snagged:
Code:
P0560 System Voltage Malfunction
P0561 System Voltage Unstable
P0563 System Voltage High


Some ECUs reset the stored fault codes after a number of ignition cycles and others keep then in a pseudo FIFO buffer where new codes displace old ones. Depending upon how yours works you may have 12yr old codes stored so you will need to get the codes read/reset, go away and blow some bulbs then return for a fresh reading.

I also have another hypothetical scenario: Lets say there is something inductive on the car's 12v supply, e.g. a suppressor choke, wiper motor or a blower motor. If there's a faulty (intermittent) feed to that inductor, when the power breaks there will be a back EMF when the magnetic field collapses around the inductor that will send a negative spike back onto the 12v supply.
I appreciate this theory sounds a bit far fetched. I just wanted to get you thinking about (or looking for) things like big suppressor chokes, perhaps aftermarket ones the previous owner fitted to prevent 'thumps' killing speakers when the headlamps (etc) were switched on?