When you installed Firmata on the Arduino, you rendered it too stupid to do ANYTHING on it's own. That includes responding in any meaningful way to a timer, since the Arduino can not push data.
was wondering whether I could convert the value of the timer into an analog output
What "value" does the timer have? What analog output would 27 seconds correspond to?
Can PureData read data from PWM pins, in any meaningful way? PWM is the only way to have analog output.
Can I convert any values into an analog output
Yes.
with an Arduino script?
You mean "with the Firmata script", and the answer to that is no.
Even if you could convert the timer's time into an analog output, and PureData could read the PWM pin to get an average voltage, how would that communicate the state of one of the switches?