A long unresolved deficiency of CHDK and SDM for SX10-IS is their failure to override either aperture or exposure when using external flash, even though these overrides work and provide correct e-TTL exposures with the pop-up flash.
I have now found with both current CHDK and SDM that prior actuation of the shortcut button in AE mode (that is separating the preflash from the main flash) solves this problem when using a 220EX in both e-TTL and "H" mode. You will of course have to assign the default Alt evocation button to either "Flash/Phono" or Video. With the 220EX don't expect adequate flash at shorter than 1/500, it's ignition time-course is too slow. This work around does however allow imposition of desired higher f-values in Tv mode set to 1/250 or higher.
With non-TTL flashes such as Vivitar 285HV. Here's a partial solution:
With the latest SDM version of CHDK. Set "Slave" on (I used setting 3).
This allows override of aperture in Tv mode by first pressing the (enabled) AE function. It does not (yet) work in Av mode.
(originally posted to http://chdk.setepontos.com/index.php?topic=6359.0
and reposted here for general visibility [sorry] )