Apparently, the best way to pass an arbitrary name/value pair string between Apple and Google contacts is through a URL field, albeit, the value is URL encoded. An arbitrary label can be assigned to such field to prevent namespace collisions and give AppleScripts a way to identify the pair and read and write to it.
Here’s an example:
Apple Contact
Google Contact
I’ll use the first “URL” with the label “Greeting”, to pass a parameter string to my Occasion Greeter AppleScript; the string supports multiple parameter name/value pairs delimited by a pipe character (“|”), which has the URL encoded value of %7C.
The second “URL” has the label WeChat and contains the text string of the contact’s WeChat ID. (The only problem is that I can’t seem to select the ID without putting the Contact into Edit Mode. 
)
Interestingly, clicking on either “URL” in Apple Contacts does NOT launch my browser, but it does on Google Contacts.
Does anyone have any other suggestions for putting name/value pairs into an Apple Contact in such a way that it syncs with Google Contacts?