A standard piston converter does not fit the retractable-nib Boheme and is not a sensible cleaning option either — the only exception is the mid-size model with a fixed nib. A proven approach is to build a cleaning tool from a thoroughly cleaned original cartridge cut open at the rear, with a pipette or rubber bulb fitted air- and water-tight; this lets you flush water across the nib and feed without flooding the mechanism. Montblanc offers a glass tube with a rubber bulb for this purpose. It is important not to let water run directly through the body, as the mechanism can corrode in the early Boheme, Rouge & Noir and Noir & Noir models. For roughly the past one and a half years the mechanisms have been made of stainless material, and Montblanc will swap older mechanisms in service on request.
Related pages on fountainpen.de:
• Montblanc Bohème — model overview: https://www.fountainpen.de/boheme-en.htm