I'd like to be able to set a trigger for a knowledge article based on a custom field.
ie. I have an article that is only good until "x" date. I create a custom field to set that date. Then I'd (ideally but not available now) create a trigger to "close" the article when "x" date arrives.
This is possible, assuming you have the custom field, you can then create a scheduled action to close the article.
First, create a custom report under knowledge to find anything that's due to expire before today.
Then, save the report, and click on "Schedule & Queue" to set up an action schedule. This will allow you to perform actions on anything that meets this criteria on a scheduled basis. See the report below, this will close any knowledge article that meets the report criteria every day at 7am.