Hi,
It should work for customer exits also but not sure exits with default value. With variant values you can make both static or dynamic.
Let's say you have 4 variables V1, V2, V3, V4. out of V4 you want to make it dynamically pick. Then check the settings of variants (if you check the option, then values for V4 will become dynamic.
Is this you are looking for?
Hope it helps!
Thanks,
Umashankar