mind you its been awhile and I don't have my manual handy.
but if you don't have apu or gpu , what is your pneumatic source to spin the starter?
and if you do have a pneumatic source, I do seem to recall that a mechanic can manually open and close the valve from outside, under direction of the pilot via intercom.
I can recall one start I did with external pneumatic, and ship's battery only...what a chore, we had to tell the mechanic when to open and close the valve. of course once one engine was running we had AC from eng gen and then did a cross bleed start.
does this help?