QtM let me know this post was up. Apologies for not visiting more often. I kinda burned out on climbing bulletin boards a few years ago, after being a true junkie for a while there. Hope you all are doing well!
As for Ogawayama, the local forecast is currently showing high temps of around 10C (which is what, around 50F?) and lows down to a bit below freezing. Here's a link, if your computer can handle the Japanese (can't remember the codes to make it clickable anymore, sorry):http://weather.yahoo.co.jp/weather/jp/20/4820/20304/3841403.html
As you'll see, temps are forecast to go up even a bit higher later in the week up there, and Tokyo is quite warm at the moment.
So temperature wise, I would think it'd be climbable, although not exactly balmy. You should be able to chase the sun around a bit, if you don't mind the hiking, which would also help.
As for snow cover, the earliest I've ever been up there to climb is mid-April, if I remember right, and at that time there were fairly substantial snow patches in deeply shaded areas, but other than that not too much problem.
Crossing the river from the campground side to the other side (where many of the climbs are) is painfully cold, and could require care if it's running high with runoff. There are several concrete spillways where things get shallow and flat. Just walk upstream along the river from the campground and you should happen on one or two.
By way of caution, I'm pretty sure the campground isn't open yet. So you're permitted to camp, and it'll be free, but there will be no amenities available.
If you need more info., feel free to mail me direct at tokyobill123 at hotmail dot com. As noted, I'm not the frequent visitor here that I once was, and so might miss a followup post. Although I'll try to remember to check.