I can't seem to get a custom command which takes a parameter to work, it just says "Error running script: Unhandled command variable 'myvar'". E.g. even a blank script gives this error:

<command>
<pattern>flibble #myvar#</pattern>
<script></script>

Am I doing something wrong?

You must use something like #text....# (for example flibble #textflip#, must start with 'text') if you want to get back a string or #object....# for an object.

ah, thanks