It already works similar to this.
You can add flag to category of items or to a specific item in items.txt.
So for example to ALL Pets or to just one specific pet, like this:
"flag" "o"
Then what you can do is give this flag only to Steam_ID that you want to have access to this item.
You can find all flags here:
https://wiki.alliedmods.net/Adding_Admins_(SourceMod) (under levels category)
If this does not solve your issue (because there simply isn't enough custom flags available to cover all steam_id) then yea we will need to configure and recompile the store script.