There is a way to do some automation; and it is not by using the list of Arch Linux packages or installing an alternative solutin, but instead by using the list of installed files.
Meet PFL; the Portage File List, which keeps track of which files belong to which packages. So, theoretically, you could download the MySQL dump of PFL database (because you don't want to flood their server) and then query your entire partition against that database. As an end result, you should have a quite extensive list; it won't cover everything, but it should cover most.
What you do now is not throw this list into Portage; there is a good reason for that, which is that you don't want to explicitly merge dependencies. So, you will want to remove anything from your list which contains
/lib to start with; that won't be enough, so you will want to do another manual run over it to ensure that it only contains the applications you actually use. Dependencies like libraries will be automatically pulled; so, they don't need to be explicitly merged. Explicitly merging them will be disadvantageous, because you are likely to pull incorrect versions.