Hi Igi,
If it's going to be possible at all, then in November when things are still fairly dry. However, when I was last in Mananara, which is admittedly now over a year ago, the only long-distance taxis brousses were on the north/south stretch to Maroantsetra and Toamasina/Tamatave with nothing going inland. These are running fairly regularly at the moment - for example three or four departures a week from Maroantsetra - so you should have no problem getting to Mananara.
If you can't cross to the west side from Mananara, you could continue up the coast to Maroantsetra (a really beautiful trip, despite the discomfort of an appalling road). You can then trek to Cap Est or Antalaha or alternatively take a short hop with Air Mad to either Antalaha or Sambava. At present there are daily taxi brousse departures from Sambava either through to Antsiranana or to Ambilobe, where you can easily pick up transport going north. (The "road" from Vohemar across to Ambilobe is in dreadful condition. The 160 or so km took me 12 hours by taxi brousse a couple of weeks ago.) Once you're on the west side, though, ground transport between Antsiranana and Nosy Be is pretty unproblematic.