Hi, if you're looking for 28mm figures, Eureka Miniatures does infantry figures for 1864 Austrians, 1864 Danes, and Franco-Prussian War Prussians. I've painted up a 28mm 1870 Prussian infantry brigade with the Eureka figures. They were fun to pain.
I know of no one who does 28mm Danish cavalry or artillery or mounted command. You could use 28mm ACW Union figures in a pinch. I'd hope Eureka would expand their FPW and 1864 Austrian and 1864 Danish lines.
I game 28mm Franco-Prussian War with Chassepot and Needlegun. There are variants for the 1859 Franco-Austrian War and the 1866 Seven Weeks War between Austria and Prussia. I think I could adapt a rules variant for 1864 Danish War if necessary.
Other figures ranges that have 28mm Austrians, 1866 Saxons, 1870 French are 25mm Battle Honors from 19th century miniatures. There also Wargames Foundry for 28mm FPW and the 1866 Miniature Range formerly of North Star Miniatures.
I hope this helps, otherwise, you might want to do it 15/18mm using Rank and File/19th Century Miniatures or 15mm Frei Korps.
Blake
looks at some new terrain products, which use space age technology!