Finally i've tested your proposal out:
1) I've installed 'Graphviz' in the winpython folder in subfolder "t\Graphviz", but the winpython.ini should be set with
PATH = %WINPYDIRBASE%\t\Graphviz\bin;%PATH%
You still will face a problem when trying to execute the hello world example:
from graphviz import Digraph
g = Digraph('G', filename='hello.gv')
g.edge('Hello', 'World')
g.view()
3) start the winpython and execute "dot - c"
After this graphviz will work in winpython.