many themes call the default icons used by windows in their bars, these tend to be quite low resolution and appear jagged, detracting from the overal presentation. Is it possible to allocate a different icon within the skin, adding to the theme?

Of course

bahaha!

my favourite method is using xtaskbar's icon substitution. (although alt-tab icons's can be a bit of a surprise)

by by far, the best method is to reshack ;)