The question of custom stocks came up when SARC first started, I was told custom stocks were allowed but if they had adjustable cheek pieces or butt pads etc they should be put in the "neutral" position.
In my head a well made custom stock (with no adjustable bits on it) is no different to a target stock that has been set up for you because both will fit you well, one is fine for sporting class and the other has to be in the target class
The other thing to bear in mind is some of us are left handed and there are not many left hand friendly rifles/stocks out there so a custom/customised stock is sometimes the only option for us. A blanket ban on custom/customised stocks would mean that some people would not be able to compete in the open class with their existing rifles, even if the custom stock does not have a deep fore end etc. Every one of my stocks (I have a lot of of rifles) has been modified in some way or another to make it more left hand friendly
I think the only way around it is to set maximum heights/depths/lengths of key areas like the hamster/fore end, length of pull and cheek piece etc.
To throw a spanner in the works, some off the shelf rifles are being sold with deeper fore ends, higher cheek pieces and better butt pads. Manufacturers are waking up to the fact that 99% of people fit scopes to their rifles, selling a stock that is designed for open sights is not a good idea (sadly some still sell rifles with the open sight mentality even though there is no way of fitting open sights to their rifles
Either way have fun deciding on the rules