You're a legend.

The Custom Filter field is exactly what I was looking for... with the delimiter/spacing workaround(s), that is.

The ability to view this in the Client List is a bonus - which I overlooked. And I've added our version of "LacerteNum" to our third-party application (formatted as ClientNo_TaxType_Year).

So I am now able to use the Custom Filter field as you mentioned, and if needs be; append to the Custom Filter string that an additional Client Note exists, and read/update the Note file if necessary (somewhat risky, I know).
Side note: the Note file that I am referring to is in the \xDATA\NOTES\ClientNo.IG9 file - in this example, opened in Windows 10 - Notepad.exe:

Thanks very much!