In practice, DL could do something (not sure what, perhaps charge the guest lounge fee?) if you make a pattern of using a lounge early based on a SDS request and then failing to take a SDS seat that's offered to you. In the extreme, they could ban you from participating in the DL FF program or from ever entering a DL lounge.

When folks have made a pattern of skipping final legs (which made their tickets much cheaper), IIRC DL (although I might be thinking of another airline) has billed them for the fare difference, giving them the choice ton pay or be banned from the FF program.