The reason why ASRock isn’t offering a custom board yet is due to the fact that RX Vega is still not available in good quantities. Manufacturers want to maximize profits with this card because custom cooling or not, users who want to buy Vega will buy it
Its news because this adds to the very-few RX Vega 64 and 56 brands/models and is being done now. The price does have to be right -- I assume $540 or less since one can get a custom card for $560 to $580.