Thanks guys, amazing tool - I used Rémi's suggestion 1) above: %e@LiteralText-%nc.%e;%f[2,"@"]
This works in the situation of paired files, like
IMG001.JPG
IMG001.RAW
IMG002.JPG
IMG002.RAW
so it covers some of my use cases.
Perhaps this thread should be moved to How-To.
Now, the other use case I am trying to solve is where there is not an exact match of JPG to RAW files:
IMG001.JPG
IMG002.JPG
IMG002.RAW
IMG003.JPG
IMG003.RAW
IMG004.RAW
For this case, the above pattern sequences JPG and RAW separately, when the desired outcome is to keep the image pairs together when both are available, and then rename:
IMG001 (JPG)
IMG002 (JPG, RAW)
IMG003 (JPG, RAW)
IMG004 (RAW)
I realize that this might be a bit of specialized use case, Siren seems otherwise quite capable.