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.

This is more of a usage question:

I am using the suggested image renaming pattern for synced renaming of JPG and raw files:

    %e-LiteralText%nc.%e;%f[2]

I would like to rename to something like LiteralText-NNN.*, tried

    %e-LiteralText-%nc.%e;%f[2]

and various variations of escape characters without luck.

Underscore works fine of course,

     %e-LiteralText_%nc.%e;%f[2]

produces expected results of LiteralText_NNN.*

Perhaps this is covered in the help text but I was unable to find it looking for 'literal' and 'escape' words.