Flash exposure compensation is readily available in P, Av, Tv modes in my a570is and obviously many others (Canon feature). In other models it's likely accessible via the Digic III propcase 127 or Digic II propcase 28 (see PropertyCase - CHDK Wiki
This means you can already write a script to do this:
rem Change flash exposure compensation setting in full Ev steps.
rem Digic III propset version, replace 127 with 28 for Digic II propset cameras.
@param a Flash Comp (Ev)
get_prop 127 P
set_prop 127 P
And no, I didn't try that script. Also, for completeness input should preferably be sanitized and prop 121 (15 in DigicII) tested.
If you need it to be more ergonomic, you should try to think of a good way to access it. Something buried deep in extra photo operations that you can pick and place to your custom user menu is probably the quickest way to improve usability.