Hi Hotfire...
Songkran is 13 Apr 2012 - 15 Apr 2012. Make sure your riding is all done before then... If I was you, I would try and hang around for it, as I reckon it is the best fun you can have standing up until you fall down - but most of the grumpy old longtermers hate it...
I would suggest maybe doing two loops out of Chiang Mai, rather than a single 7 day loop - Maybe May Hong Son (East of CNX) - 2 or 3 days, and than Loei/Nan (West of CM) for 3 or 4 days...
That way you can carry less gear, get back to Chiang Mai civilisation, and have some Western Food... if you are having any trouble with the bikes you can swap them....
Loei, is just inside the Isaan region, (Nan is just outside) which is culturally and geographically very different to Mae Hong Son, which is more closely ethnically and geographically to Burma, just across the border...
IMHO, the riding is better to the West of Chiang Mai, but the scenery better to the East...
Note that by March, there has probably been no significant rain for 6 months, so aside from being freaking hot, the scenery will be mostly browned off (gets that way from Jan)... Also, The Northern Thai's love a bit of slash and burn farming, and in March apparently it is a big problem...
Hope this helps...
Cheers,
Darryl