"This brought up a feature of excessive write to storage in particular: such writes might prematurely wear out the SD card."
I have done a fair amount of work on this very same thing for my machines with SSDs. I submitted a SSD saver patch on RH bugzilla here:
In general, booting up produces several MB of write. I have reduced this to essentially zero by putting things that don't need to be persisted to tmpfs, e.g.:
/var/log (only on laptops)
There are a lot of additional things that are on tmpfs, depending on the application of the machine (e.g. browser caches).
I also disable swap in favour of compressed RAM as swap:
Other things you may want to check out for lowering CPU power consumption is undervolting via PHC. Intel chips have a LOT of margin for error in them, and I have generally achieved peak TDP drops of up to 25% on Core2 class CPUs with no loss of stability (verified by days of OCCT and other testing). Have a look here:
All the functionality is already built into Intel processors since the Pentium M, you just have to leverage it properly.
You also mentioned CPU governors and on-demand one specifically. Conservative CPU governor generally yields much better results in my experience.