The closest thing I could find to the suggestion in the OP was in Brand Martin, *Japan and Germany in the Modern World*, pp. 25-6.
"Consul Brandt, at his forgotten outpost, thereupon took to outlining highly promising colonial projects with which to assert his own influence on the then topical issue of whether Germany was to have colonies of her own. Since the Prussian expedition, German naval circles had secretly favoured acquiring Formosa as a colony. Brandt, who knew about this, tried to shift the German interest away from the subtropical island and towards the northern Japanese isle of Hokkaido. In two voluminous memoranda he tried to convince Bismarck and the admiralty staff of the natural advantages of Hokkaido and outlined plans for taking possession of the Japanese island by sending a 5,000 man landing force together with between six and eight gunboats. With typical early imperialist thinking, Brandt justified his plans for annexing Japanese territory by labelling the Japanese 'culturally incapable' and unable to cultivate the island by themselves. When the government in Berlin did not respond to these theoretical reflections, Brandt took action on his own and tried to create a fait accompli.
"During the turmoil of the civil war, the Prussian representative supported the Japanese opposition groups whom he supplied with arms. This did not prevent Brandt, after the victory of the South, from engaging in talks with one of the defeated daimyos (feudal lords) from the North about selling their land to the Prussian Crown. In order to prove that Hokkaido was perfectly suitable for German settlement, with the help of the Prussian Honourary Consul in Hokkaido a 100-hectare area had been turned into an experimental estate - 'Augustenfelde' - and had been taken on a 99-year lease. Bismarck, however, reminded the consul that the treaty powers, including Prussia, had declared their neutrality in the civil war. The Prussian navy was not altogether opposed to the project, but considered the present time as inappropriate. *However, in case the Japanese Empire collapsed the navy would be there to take its share.* [emphasis added--DT] Although his Hokkaido project had been turned down, far from giving up, Brandt, together with the geographer von Richthofen and the navy, pursued his colonial aims on a smaller leve1.7 He now looked for any islands which would be suitable for a naval base and which were outside of the new Japanese government's sphere of influence. Yet, Bismarck did not hold with these plans either. He considered the German navy too weak to 'stand its ground if in case of war it were to be seriously attacked by the enemy's probably superior armed forces'. The hazardous imperialist policy of the first German diplomat in Japan was thwarted by the unexpected ending of the civil war and by Bismarck's fundamental unwillingness to engage in colonial adventures."
https://books.google.com/books?id=7_jBOpYASMQC&pg=PA25
So, a few observations"
(1) We are actually talking about Prussia acquiring Hokkaido at a time prior to German unification--and indeed before the Japanese Emperor had consolidated his authroty in the Boshin War.
(2) The idea was that of a mere Consul (Brandt), and had no chance of getting Bismarck's approval. Obviously Bismarck thought German unification had to take precedence over colonial advantures in the Pacific.
(3) Brandt was not under any illusion that the Japanese central government would sell Hokkaido--he wanted to take it by force. Later, he did propose to "buy" it--from the defeated daimyos of the North. But they were after all *defeated* and were in no position to sell it. In any event, the short-lived Republic of Ezo
https://en.wikipedia.org/wiki/Republic_of_Ezo was under the guidance of French military advisers, who would not be very likely to look with favor on a sale to Prussia.
(4) Once the Emperor had consolidated his power, there was no chance of Japan selling Hokkaido to the Germans or anyone else. Whoever wanted it would have to take it by war.
Basically, the only (very slim) possibility for a German Hokkaido is indicated in the sentence from Martin that I have emphasized: "However, in case the Japanese Empire collapsed the navy would be there to take its share."