P is discount business, so on its own it is not upgradeable (as you seem to be aware). If you are Diamond on ANA, you can use Upgrade Points (not miles) to upgrade normally non-upgradeable fares using twice the number of Upgrade Points that gets cleared day of. Not a great value, but in my experience (4 times over various winters) it always clears. The requirements SEEMS to be that there is just an open seat and food on the day of departure. So easier than an open O fare.

For business to first the cost is normally 20 upgrade points one way, so the cost is 40 upgrade points via this method. If you barely hit ANA Diamond, you can expect 60 upgrade points, so this eats up those points quickly. Of course if you fly more and have an ANA Japan-issued credit card, then you could rack up more upgrade points but even so the maximum would be 120 (100 from flying 250K premium points, 16 from Diamond benefits, 4 from credit card).

I’m not sure the same day or few days before strategy works too well economically, since fares are much much higher close in, but in theory it could work. ANA does open up sometimes F inventory a few days before departure.