Indeed a good question. I just took a look at the electoral law of Schleswig-Holstein. In article 3, section 1 it is stated that "parties of the danish minority" are excluded from the 5%-clause. The SSW is not specifically mentioned. But itīs noteworthy that the plural is used ("parties" instead of "THE party of the danish minority").
Iīm not totally sure about this, but I would say that in the case that another danish party is founded, itīs up to the election supervisor of the state to decide. If he or she refuses to grant the party this special status it would probably end up before the court.
Ironically, I met CDU members who already claim that the SSW is just a danish-speaking "spin-off" of the SPD.
This system could have a tendency to lock Danish speakers in. They either have to accept the political ideology of the SSW, or vote for a German party.
I wonder what would happen if they followed the New Zealand model. As I understand it, Maori's can choose to be on the national voting role or the Maori voting role. Constitutiencies are defined based on the number that choose each. I think one of the Maori constituencies is the entire South Island.
In a proportional system like Germany, Danish speakers could choose a Danish ballot, and the first allocation of seats would be based on the number of voters who voted with a Danish or German ballot. Then the seats would be allocated separately for the Danish and German groups. The threshold (5%?) could be maintained since it is likely that several parties would achieve 5% of the Danish vote (there might a higher effective threshold because of the small number of seats). Candidates could be formally affiliated with the German parties, or they could run on an separate Danish party.