\parindent=0pt
\pdfprotrudechars2

H

\font\x=FdSymbolB-Regular at 10pt
\x
\lpcode\x`b=500
bye % no protrusion

\fontdimen6\x=10pt
bye % protrusion fixed

% here it doesn't work
\letterspacefont\xy\x 300
% \fontdimen6\xy=10pt % no difference
\lpcode\xy`b=500
\xy
bye
\end