I don't think there's a serial command to insert/append. In order to do that, you have to communicate to the player via the emplode connection protocol. Don't think Hijack and the web server can do that.

Empeg guys: Hint: It would be really nice if we could have an insert command from serial similar to the # command. Perhaps i#<fid> and a#<fid> or something? Dunno?

I'm going to wish-list this to the guys now...