I did two weeks on Hokkaido back in August, and probably the best of it was north of Sapporo anyway. From what I hear of the disaster around Sendai, it could take them a while to rebuild that area, and it may not be safe for quite a while, so if you're absolutely determined to bike on Hokkaido, I'd think about flying into Hakodate and riding the length of Hokkaido from there. If you head from Sapporo to Yubari and then head north toward Furano, you'll pass through some really pleasant areas -- quite possibly the most remote in all of Japan, then heading through the lavender fields around Biei, and maybe some time in Daisetsuzan National Park (which I missed because a landslide cut the road).

If the weather up that far north isn't agreeable in April, you could explore some of the area south of Tokyo, too.