Parse order? Using a plugin call inside weblog:entries call

I have a plugin that retrieves some category id’s and returns them separated by pipes, like 1|2|3|4 - this works fine when I call it on its own in a template.

However, when I try this:

{exp:weblog:entries weblog="videos" disable="categories|category_fields|member_data|pagination|trackbacks" rdf="off" category="{exp:my_plugin}" dynamic="off"}

it does not work - the page is blank. Is there any way to get this to work?