And now there is a new option. I just created a pscriptor Numberize scriplet (attached):
# Returns a sequence of numbers defined by the arguments passed via -a. A starting number
# and possible increment can be supplied (both default to 1). The counter restarts at the
# beginning of each group, if defined. The format of the -a argument is as follows:
#
# -a "seq=<start number>+<increment> field=<grouping field name>"
#
# where <start number> is the starting number of the sequence, +<increment> is the increment
# to be added to the previous value (the +<increment> is optional, and defaults to 1). The
# <grouping field name> defines the grouping of files, where withing each group, the sequence
# of numbers restarts at the <start number>. If the field=<grouping field name> is absent,
# the grouping will be the entire selection.
Download and see it live:
https://dl.dropboxusercontent.com/u/87189402/Numberize.mp4