With the Family Upgrade, I think you can use the DLC on any account that uses the same disc to play the game, as the account that it was downloaded for. I'm not 100% on that though. Someone else will come by and clarify that for you, I'm sure. 👍
Yes, I can confirm:
I have a EU main PSN account (Fr) and a JPN secondary PSN account (that I have created years ago when the first GT4 JPN demo version was available) and now I play on an EU (FR) GT5 PAL version with both;
I have bye all the DLC from beginning and all are available in my JPN account![]()