Changes

\setuppapersize[A6]\define\thisisthestart% {\grabbufferdata[mybuffer][thisisthestart][thisistheend]} % If you want to perform some action when the buffer ends% \let\thisistheend{...} 

My friendBent double, like old beggars under sacks, you would not tell with such high zestTo children ardent for some desparate gloryKnock-kneed, coughing like hags, we cursed through sludge,The old Lie: Till on the haunting flares we turned our backsAnd towards our distant rest began to trudge.\rightaligned{--- Wilfred Owen, excerpt from {\it Dulce et decorum est Pro patria mori.}}

\typebufferobeylines\getbuffer[mybuffer]