Some theories on how to approach this:
Projectile speed has to be zero. If not, it would keep on moving forward until its lifetime has depleted, distorting what you wanted to do. Set muzzle_velocity = 0 for the gun you want to use.
Damage of the gun has to be zero, too. Otherwise, if you're touching the station while spraying it might become hostile toward you. Set hull_damage = 0 for the munition of the gun you want to use.
Set the lifetime of the munition to some value you're comfortable with while spraying. Too low a value and it will start disappearing before you're finished spraying; too long and you'd need another station to spray on, because the old one would be "full"
This is done by modifying the lifetime = entry for the munition of the gun.
Refire rate. The higher it is, the more continuous your spraying will look. Set the refire_delay = value for the gun of your choice to something you like.
Of course while spraying, you have to stand still, not flying at any speed. Turning your ship however should still be possible.
Like I said above, this is just theoretical. If it works, please let me know.