Thanks,
Don't use the "Pipe text through" option. Instead, create a new command, scoped so it will only be triggered within the context you want, with the key shortcut ⌘⌥⌃P (that's cmd-opt-control-P). The output format should be View As HTML. This way you can press ⌘⌥⌃P and get what appears to be a web preview, but is really the results of your command.
This I did before and it works perfectly.
But my point is the nice Web Preview option "Refresh after change".
That means that while typing my document the transformed Web Preview shows the latest changes. Of course I always can type some short cut to get this via a command, but "Refresh after change" would be more comfortable ;)
On the other hand, maybe there is a way to call a Bundle's command like a cron job, i.e. a command which will be execute e.g. each 0.5s.
Thanks in advance
-Hans