Looks really good!

    F:"\n"/:0::;C:,[;"\n"];I:`:index.txt;f:-<I;h:C F I;H::F`:chat.htm
    .z.ws:{s:(" "/:($"du"$.z.Z),,x);{x y}\:[-!.z.W;s];h,::C s;f s}
    .z.po:{(-.z.w)h};.z.ph:{H}

1. "\n"/:0:: occurs twice and can be made a function (F)

2. You can project the `y` argument by itself (C)

3. Shorten symbols (I)

4. No need to (-f) all over the place when -< will do. (f)

5. You can load chat.htm once (H; .z.ph), but unfortunately K4 won't notice changes to the file, so am a bit divided about this one.

6. Save brackets! Join-enlist (,,), negate til (-!), and amend (,::) (.z.ws)