Q:

Fermenting in Sealed Keg

I have heard of people fermenting in corny kegs before. I've also heard of people fermenting under pressure. Finally, I've heard of people fermenting in corny kegs under pressure, using a spunding valve.
What would happen if I just...transferred the beer, oxygenated, and sealed the keg? A corny keg can almost certainly handle the increased pressure. (I carbonate at 30 psi...). Would the pressure be too high? Would the pressure harm the yeast?
I felt almost certain I'd find the answer here already, but I don't see anyone who's already asked it...

A:

The effects of pressure on fermentation have been pretty well studied. As stated in your question, these studies all seem to use a spunding valve that releases CO2 when the pressure goes above a certain point, ~30 psi or less. In this answer I will run some numbers to see what might happen if that pressure is not released.
This website did calculations for how much CO2 a batch of beer will produce. The page explains their methods in detail. More CO2 is produced in beer that attenuates more, and in higher SG beers. A 5 gallon batch of a middle of the road beer might have an SG of 1.050 and 75% attenuation. It will produce roughly 1.6 lb of CO2, 13 cubic feet or 16 moles.
A typical corny keg has slightly more than 5 gallons of space. This answer suggests 5.25 gallons, but doesn't back it up with anything. For the sake of argument, lets assume a quarter gallon of head space, 9.46e-4 m^3. Note that the total volume of unpressurized CO2 produced by our example batch, 13 cubic feet, is roughly 100 times larger than this space.
The ideal gas equation is P = nRT/V, pressure equals the number of moles of gas times the universal gas constant times the absolute temperature divided by the volume. Squeezing that much CO2 into that little space requires roughly 6000 psi of pressure. This is a completely unreasonable amount of pressure. 
I made a google sheet that you can copy and edit if you want to play with numbers, however only the very low end gives you anything that even resembles 30 psi. A 1 gallon batch of beer would leave you 4.25 gallons of headspace. If you reduced the SG to 1.030, you would wind up with roughly a tenth of the CO2 of the example batch. This comes out to a pressure of 36 psi.
I don't actually believe that you will wind up with 6000 psi under any circumstances. One of the following things will happen instead.

Your keg explodes (or pops a leak). My keg has a warning on the lid not to exceed 130 psi. I would guess that there is some safety factor built in; maybe the keg won't break until it gets to twice that. The keg will not last to 1000 psi, much less 6000. 
The yeast gives up. The pressure gets too high for them, and your fermentation stalls out before completing. Neither option is a good thing.

