UKH

I am guessing Mt. Fuji in a day is unlikely from Shinjuku especially in terms of transport logistics. How about going up in the afternoon and bivvying high up, would this be tolerated in high season?

Would be going end of August.

I realise Expedition & Alpine is probably not the right forum...

You can definitely get up and off it in a day - and if you started early from Tokyo and made sure you checked the public transport back you should be able to get back to Tokyo OK as well.

I think I'd be nervous bivvying on an active volcano, even if the last eruption was in 1708. Could have its advantages though; the descent may not take as long as you were expecting. 

My son is on his way back from Japan right now and I’m pretty sure he did Fuji in a day from Tokyo.  I’ll be able to ask him tomorrow.

I thing Karl Pilkington did it in a day and didn’t enjoy it .

Can't remember how long the drive is from Tokyo but certainly Fuji won't give you any bother in a day. Don't forget to run down! 

Only time i've done it we travelled the night before, then summited for sunrise. Obviously depends on the time of year, but it can get incredibly hot and humid if you climb during the day. Be prepared to be overtaken by spritely octogenarians!