Not sure if people have attempted this, but if you draft an Ohtani, couple that with a common or bronze fielder later in the draft. Take a fielder that you like and put him at DH, take the common or bronze player and put him in that players position on defense and place him in the order where you want Ohtani to hit... then when the game starts move the DH to his position which forfeits the DH allowing Ohtani to hit... yes it's quite a bit of manuevering but that's all I got until SDS figures it out. I know in the regular part of DD you have to go to DH slot and when you go to sub on your Manage Squad screen the available Ohtani cards that can DH will be available even if you already have him in your starting rotation.
Also side bar: It looks like they fixed it where if you have multiple cards of a player you can now see the card available as a sub.
I know at launch in order for me to switch from Captain David Wright to Rookie David Wright, I had to put another card at 3B and then go back in to put in Rookie Wright.
Last night I had WBC Cruz (Padres) in the DH spot for my NMS team and when I went to the sub menu the TA Cruz (Orioles) was available as a sub... I think ultimately I decided to stick with WBC Cruz but its nice to know that I don't have to do a workaround for that anymore, not to mention I can easily compare the difference in the two cards now.