@Rémi, @Stefan:
Thanks for your answers and hints. It took me a second to understand the parameters; having it explained this way is far better than trying to understand helpfiles. However I have some comments. As Stefan wrote, the chances of creating pseudo-duplicate filenames is high. I say "pseudo" as they are not true duplicates but simply incorrectly handled files.
Here the possibility of completely switching the parsing of Siren from ascending to descending (renaming highest-numbered files first instead of starting from the lowest) would avoid having to use dirty workarounds, as the one suggested by Stefan. As indicated by Rémi Siren proceeds per selection order ... nobody likes to manually select fifty or more files just to operate top-down instead of bottom-up processing so a hardcoded switch in sorting order (asc/desc) would be required here. I am sure that there are more operations that would also profit from this, be it only by reducing steps in larger jobs.
I had a closer look at the help file and I can not seem to find a generic "until filename end" variable. Sure, using %f(-11) inserts the last eleven characters of the filename, but that creates havoc if filenames include additional comments, as in
LName (PName) 21-200-01 (1945, Museum).txt
LName (PName) 21-200-02 (Citation Website).txt
LName (PName) 22-200-01 (1964-JAN-12).txt
The only way aound that (as far as I can see) is to manually count the leading letters including the counter element, then add that into the brackets. Per example "LName (PName) " that would be %f(17). Not particulary nice nor user friendly, but appears to work. Have I missed this function or is there really no other way?
Thanks for all your help!