You can extend autoexposure Tv range by writing a script that first determines exposure at widest aperture, highest ISO and then uses APEX formulas to calculate Tv for your selected Av and ISO. Obviously the same applies to the short Tv range by going into half shoot with minimum aperture, lowest ISO. I believe you don't even need multiple half shoots as you will be setting Av, Tv, Sv as overrides during half press after autoexposure.
The simplest way to write the script is probably to require settin the camera to these incorrect Av/ISO setting before starting the script and giving the actual desired Av and ISO as script arguments.
But do note that there is a reason they limited autoexposure to 1 second. When it's too dark even for wide aperture & high ISO, the camera can't tell black from even more black as it just sees some noise. That's where the shot histogram method found in sunset timelapse scripts (they analyze the previous shot's RAW buffer instead of the noisy live view image autoexposure works with).
