Contacts Recommendation

The photo field is generally used to store a photo of a persons face, as a person normally only has once face there is normally only a justification for one such field.

(Since some people are described as two-faced I will leave open the possibility that a few people might legitimately need two fields. 😉 I don't think anyone these days is going to need a contacts address card for Janus the Roman god of beginnings, gates, transitions, time, doorways, passages, and endings.)

I therefore do not think Apple are likely to provide this feature. It is also worth bearing in mind that the Contacts program can be and often is linked to various corporate directory systems and these also are limited to a single photo - for presumably the same reason as described above.

The nearest workaround I can suggest is having two separate contacts for the same person with most of the details the same. This is something possible with the Contacts program.

There is a notes field available for each contact. You could put a URL in that field which takes you to a webpage which then displays the relevant bar code.

While driving this via a database which generates the barcode dynamically would be possible and clever, a simpler more basic approach would be to store each barcode as a PNG (JPEG might not be good enough quality) and the URL would refer to the unique file name of the PNG file.

Note: A client Mac can still act as a web server although this might be easier with a copy of Server.app or some other 'real' server.

I don't know what business you are personally in but you should 'sell' them a WiFi solution.

Another similar workaround,

You can drag the file into the URL field on the card. You'll have to add "file://" in front of the path so that the start of the URL is "file:///" (three slashes).

Clicking on that link will open the file in Preview.