The custom helmets doesn't have a 1 - 20 limit iirc, the 1 - 20 applies to games 'preset' helmets that the 'Reserve' drivers use since they don't have their own dedicated helmets. And I don't recall what design they used, but its same one for all of them just recolored.
Back to the custom helemts...
Go in these design slots right. You want to make a new design that the driver will 'use' and have the modded helmet assigned to it. I can't say what ERPs the helmet textures are stored, as I haven't done helmet texture modding, but its whichever ERP is used for player helmet texture mods. Since that is what we are technically assigning to other drivers.
But once you equip a helmet to one these design slots. You can then equip it for yourself, and jump into a throwaway GP mode using the custom team to check the slot ID for the player using the GP mode script, then go back to career mode and assign that slot ID to the driver you want to see using the modded helmet from the design slot.
PS. Forgot to mention, but sometimes, if editing the helmet slots when a weekend in progress. It might appear completely white without any design... It
should have a design the following weekend tho. Emphasis on 'should' as I've done very limited testing on this.