Tube screamer fan eh?
Well, do you wish to modify the medal so that it will ALWAYS be on, or just so it's default state is 'on' when its powered up? The first option would mean that, if you use it 'outside' of your rack, you'll never be able to bypass it.
Good call on the schem and PCB though, makes it far easier to see 'how to mod'.
Like almost all active bypass pedals, the audio is routed through JFET's, controlled by a two-transistor monostable multivibrator. Latch on, latch off style.
The simplest way I can think off is to swap the wires connecting to D13 and D14. You can either do this on the 'function switch PCB' at the bottom left of the schem, or on the 'output PCB'. I'm assuming these two PCB's are connected via a two-pin plug (P410 on the schem), so simply swap these connections.
Why? Well when one JFET is on, your 'clean' single is sent to the output (in bypass), when the other is on, your 'dirty' signal is sent. When you first power the pedal, the multivibrator defaults at a state when the bypass JFET is on and the other is off. Swapping the connections between these means the default is 'on'.
I'm pretty sure that'll do it, plus, no modifications to the actual PCB's required, just the connector, so its completely reversible
Blueteeth
Ps. Your English is better than mine.