A little research in the Register of Deaths, and use of the Grytviken Explorer, show that there are three possible occupants of Grytviken grave C2.

Grave C2 lies between Graves C3 (15 April 1928)  and C1 (3 November 1930). Photographs of the latter funeral show that the grave was already there.

According to the Register of Deaths, three people died in this period:

Further information in the Magistrate's records at the Scott Polar Research Institute indicates that Nilsen worked at Prince Olav Harbour and died in Leith hospital, so was unlikely to have been buried at Grytviken. Hillestad died in Stromness Bay and was treated by the Leith doctor. This leaves Marilainen, who we know died at Grytviken, as the most likely person to be in grave C2.

UPDATE October 2014

This is confirmed in THIS ARTICLE