I did a trip down that way last year, well in March / April, so nearly two years ago now. That was in a 4x4 vehicle though, so a bit less challenging than you guys, but can maybe give you a few pointers. Bear in mind that you will be there pretty much in the rainy season and what might be a fairly good road when dry will be mush if you get heavy rain. The road up to the Ngorogoro crater from the park gate is one of those so be careful. The crater itself is one of the most jaw dropping, smack in the face, pieces of natural beauty I have ever seen, so it's definitely worth the effort getting up there. Even if the park fees are a rip off and the camp site (Simba A) is crap. I also doubt it if they will let you into the crater on bikes, nice if you could let us know about that though.
The road down from there is gravel most of the way but they were working on it and should be ok. When you get to Dodoma you can decide to go tar to Iringa via Morogoro or gravel/mud/mush the shorter route straight down. I chose the mush, thats just my nature, but I was quite lucky that it didn't rain. You should be ok on that road also if it doesn't rain. From Iringa to both the Zambia and Malawi border is good surface. In Malawi the main road from North to South is newly tarred and good condition, although if you might want to stick by the lake if you're after the adventure of it.
When you cross into Zambia from Lilongwe side and looking for a place to stay check out Mama Rulas in Chipata, awesome spot. From there to Lusaka it's mostly tarred surface all the way. Also a nice place to stay in Lusaka is Eureka campsite on the road south. The Botswana part has been covered a bit already, so good luck and let us hear of your experiences. Definitely a trip I'd love to do again but this time by bike.