You are going to have flat (low CO2) beer pretty soon. Just like half a bottle of coke put back in the fridge will the next day be a lot flatter than when you opened it. Even flatter if you drank half of what was left and put the remaining 1/4 back, a day later flatter again.
If you pressurised the 1/4 full bottle with N2, it wouldn't get any more fizzy, the CO2 in the head space isn't pushed into the coke by the N2. In fact they basically ignore each other. Look up Mixed Gas Law!
There is a good reason why people use mixed gas and why there are a range of them on the market. You need to match the desired amount of dissolved CO2 in the beer, for an Irish Stout somewhere in the 3.5-4.5g/L (2 Volumes+/-0.3V) to get that you need the temperature of the beer and the applied pressure (gauge pressure, or pressure above atmospheric)
Say your beer was at 5oC, 50kPa of CO2 would give you 4g/L.
(see here)
If your mixed has was 50/50 you would need 100kPa of the mix to get 50kPa of CO2, if it was 70/30 you would need 117kPa to get (and more importantly keep) the same amount of dissolved CO2.
Naturally as N2 is less soluble than CO2 you need the higher pressure to push enough N2 in to solution so the more N2 you want the lower the CO2 mix you choose. Without the stout restrictor (the little perforated plate in a stout tap) at those sort of pressures the beer would be bloody hard to pour. the stout restrictor slows the flow and the turbulence knocks a lot of the N2 out of solution, that and its low solubility means lots of small bubbles and the classic Guinness pour.
Personally I think at a home brew level its more trouble than its worth, N2 guts the beer of most of its hop (and some other) flavours/aromas, its expensive to set up, the gas is much more expensive and you need a dedicated tap.
Personally I would go for a cask with an aspirator and drink my stout the really old school way, because it tastes better.
Mark
