It need's to be well south of 1040 in order to be drinkable. I don't like the idea of too many small additions. Get it in there and finished. Eventually your going to have an environment that is just too hostile for continued fermentation. Remember, alcohol is toxic the more your yeasties create in there the faster it will make them go dormant. Trying to restart in a high alcohol environment is guaranteed to frustrate.
Agreed that re-starting is difficult, but if the additions are diligently added to maintain the highest possible level of fermentation, then you can at least stop making additions when it is apparent that the yeast are petering out.
I think the inhibitory alcohol concentration would be attained with either large or small additions assuming one can make additions at the right time to maintain peak yeast activity (caveat). The smaller additions would have to maintain a sugar concentration in solution that is excess of the maximum rate at which yeast can catabolize the sugar.
An additional consideration is that high wort sugar concentrations can inhibit yeast activity because of the high resulting osmotic gradient, so making smaller additions
may be beneficial in this regard.
Jon,
Your yeast have a tough task ahead of them due to the detrimental effects of alcohol, CO2 (and low pH), sugars, and other metabolic bi-products, plus the depletion of free nitrogen and nutrients! Of these factors, you are already controlling nitrogen, nutrients, and sugars. Have you thought about periodically de-gassing the beer with a wine whip? Adding O2 displaces a little (but not all) CO2 from solution, but may result in oxidation over time.
De-gassing the beer will also temporarily increase the pH which could help spur yeast activity. If fermentation drives the pH much below 3(ish), then adding potassium carbonate to buffer pH may also have a beneficial effect on reducing yeast stress. This is done frequently for meads and wines.