MicrosoftI'm back to respond to my own question posted here on Jan 11, 2022 for any who might be interested in an answer.
For the below to make sense, please see that post for my original question.
Answer: Rather than attempt to install the combined SSU / LCU package twice, the SSU can be extracted from the combined package, and, if present, the SSU can then be installed as a separate entity just as it was in days prior to the SSU / LCU being combined.
NOTE 1: The "f:" in the following command is NOT a drive letter. Don't change it.
NOTE 2: The DISM command below illustrates applying the SSU to a WinRE.wim, but the same same applies to a mounted boot.wim and install.wim. The whole point is simply that the SSU can be extracted first, then applied if it is present. After that the LCU can be applied.
expand "C:\WinUpdates\LCU\*.MSU" /f:"SSU*.cab" "C:\Project\SSU"
DISM /Add-Package /Image:"C:\Project\WinRE_Mount" /PackagePath="C:\Project\SSU" /LogPath="C:\Project\Logs\dism.log"
