I'm looking at the CHDK Enhanced Photo Operations menu of my SX50hs right now.
If I set Override Tv type --> [ Ev Step ] then I can set EvStep Value [ * ]l1/100K] (with steps at 1/80000, 1//64000, 1/50000, 1/40000 etc).
If I set Override Tv type --> [ ShrtExp ] then I can set Short Exp.Value [ * ][ 0.00001]
Note : when you are entering ShrtExp values, the zoom level located at the camera's shutter button allows you to move the entry digit cursor to the left and right in the time value entry field.
What do you see when you try this?