Which PSP DLC do you mean?
If it's anything like the Vita or PSP games going onto the Vita I suppose all you'd need is a PS3 on the same PSN account as the Vita, and do the qCMA USB trick. I've been able to get a load of game stuff on the Vita while keeping it on the old exploitable firmware this way. So not through TN-V directly as it uses 6.60 FW, while PSP PSN requires 6.61.