\setupbodyfont[dejavu]

\definefiller[test-a][alternative=stretch]
\definefiller[test-b][alternative=symbol,symbol=!]
\definefiller[test-c][alternative=rule,height=.1ex,leftmargin=.5em,rightmargin=.25em]

\starttext

    text\filler[test-a]text \par
    text\filler[test-b]text \par
    text\filler[test-c]text \par
    text\checkedfiller{<nothing>}text \par

\stoptext
