As I need to use \afterGrace to be able to put the grace at the end of barline, I lose the automatic formatting given by \acciaccatura.
I think that I should change the stencil of the Flag but I don't know which are the possible values. I've looked up here:
\version "2.18.2"
%% Acciaccatura at the end of barline
\relative c'' {
s2 \afterGrace b {
\once \stemDown
% how to get a stroke on the flag?
%\once \override Flag.stencil = #???
a8
}
g
}