Is it possible to set a single member of a complex-variable inside the variable table I want to use Drush vset to modify a subset of values stored in the 'variable' table's theme_themeName_settings record. 

I've found references to vget and vset but all I've found so far is for the entire complex-variable.

Any help would be appreciated.
Drush does not support modifying elements inside a complex variable directly.  You could instead use php-eval to do this:

$ drush php-eval '$v = variable_get("theme_themeName_settings"); $v["key"] = "value"; variable_set("theme_themeName_settings", $v);'