What exactly do you want to do? What do you mean by "filter keywords"?
Do you want to find all images with a certain keyword attached in several projects at once?
In Aperture select your projects in the Library Inspector ("source list" in iPhoto) and click the loupe icon beside the search field in the Browser ; now you will see a smart settings HUD. Enable the checkmark beside the "Keyword" rule and set it to "keyword is"; mark the keyword you want to use.
P.S. You could do the same by collecting all related projects in a folder and adding a smart album to this folder with the same smart settings. Or if you want to search the whole library define a smart album with the whole library as the range.