[uvdiv]

"Ok so then there is the question of actively regulating the temperature of the ocean (which we might want to do for other reasons like food production). One might speculate on using the planet mantle as a ginormous heat sink. It has a fairly large heat carrying capacity over all."

Not on these scales. At 10^18 W (your 100 MW person * 10 billion people) -- about 5x the current solar budget or Kardashev I -- you could heat the entire oceans 10 °C in about a year, or the entire earth's crust 10 °C in about 100 years. (Oceans are about 10^21 kg [1], crust is between 10^23 - 10^24 kg [2]; I'm using the specific heat value for granite here [3]).

[1] http://en.wikipedia.org/wiki/Hydrosphere

[2] http://en.wikipedia.org/wiki/Continental_crust

[3] http://www.engineeringtoolbox.com/specific-heat-capacity-d_3...