Full credit is due to AlexG who incidentally en passant posted a solution to this problem here, which, for completeness sake and so it doesn't get lost (!), I quote below.

Relevant to the above question is the trimming option described in the man:

Usage examples:

#default operation

pdfcrop.sh orig.pdf cropped.pdf

pdfcrop.sh -m 10 orig.pdf cropped.pdf

pdfcrop.sh -hires orig.pdf cropped.pdf



#trimming pages

pdfcrop.sh -t "10 20 30 40" orig.pdf trimmed.pdf

Content of pdfcrop.sh:

#!/bin/bash



function usage () {

  echo "Usage: `basename $0` [Options] <input.pdf> [<output.pdf>]"

  echo

  echo " * Removes white margins from each page in the file. (Default operation)"

  echo " * Trims page edges by given amounts. (Alternative operation)"

  echo

  echo "If only <input.pdf> is given, it is overwritten with the cropped output."

  echo

  echo "Options:"

  echo

  echo " -m "<left> [<top> [<right> <bottom>]]""

  echo "    adds extra margins in default operation mode. Unit is bp. A single number"

  echo "    is used for all margins, two numbers "<left> <top>" are applied to the"

  echo "    right and bottom margins alike."

  echo

  echo " -t "<left> [<top> [<right> <bottom>]]""

  echo "    trims outer page edges by the given amounts. Unit is bp. A single number"

  echo "    is used for all trims, two numbers "<left> <top>" are applied to the"

  echo "    right and bottom trims alike."

  echo

  echo " -hires"

  echo "    %%HiResBoundingBox is used in default operation mode."

  echo

  echo " -help"

  echo "    prints this message."

}



c=0

mar=(0 0 0 0); tri=(0 0 0 0)

bbtype=BoundingBox



while getopts m:t:h: opt

do

  case $opt

  in

    m)

    eval mar=($OPTARG)

    [[ -z "${mar[1]}" ]] && mar[1]=${mar[0]}

    [[ -z "${mar[2]}" || -z "${mar[3]}" ]] && mar[2]=${mar[0]} && mar[3]=${mar[1]}

    c=0

    ;;

    t)

    eval tri=($OPTARG)

    [[ -z "${tri[1]}" ]] && tri[1]=${tri[0]}

    [[ -z "${tri[2]}" || -z "${tri[3]}" ]] && tri[2]=${tri[0]} && tri[3]=${tri[1]}

    c=1

    ;;

    h)

    if [[ "$OPTARG" == "ires" ]]

    then

      bbtype=HiResBoundingBox

    else

      usage 1>&2; exit 0

    fi

    ;;

    ?)

    usage 1>&2; exit 1

    ;;

  esac

done

shift $((OPTIND-1))



[[ -z "$1" ]] && echo "`basename $0`: missing filename" 1>&2 && usage 1>&2 && exit 1

input=$1;output=$1;shift;

[[ -n "$1" ]] && output=$1 && shift;



(

    [[ "$c" -eq 0 ]] && gs -dNOPAUSE -q -dBATCH -sDEVICE=bbox "$input" 2>&1 | grep "%%$bbtype"

    pdftk "$input" output - uncompress

) | perl -w -n -s -e '

  BEGIN {@m=split /s+/, $mar; @t=split /s+/, $tri;}

  if (/BoundingBox:s+([d.s]+d)/) { push @bbox, $1; next;}

  elsif (//MediaBoxs+[([d.s]+d)]/) { @mb=split /s+/, $1; next; }

  elsif (/pdftk_PageNums+(d+)/) {

    $p=$1-1;

    if($c){

      $mb[0]+=$t[0];$mb[1]+=$t[1];$mb[2]-=$t[2];$mb[3]-=$t[3];

      print "/MediaBox [", join(" ", @mb), "]

";

    } else {

      @bb=split /s+/, $bbox[$p];

      $bb[0]+=$mb[0];$bb[1]+=$mb[1];$bb[2]+=$mb[0];$bb[3]+=$mb[1];

      $bb[0]-=$m[0];$bb[1]-=$m[1];$bb[2]+=$m[2];$bb[3]+=$m[3];

      print "/MediaBox [", join(" ", @bb), "]

";

    }

  }

  print;

' -- -mar="${mar

}" -tri="${tri
[*]}" -c=$c | pdftk - output "$output" compress