Yes it's awkward to do this with dd today. We're considering adding skip_bytes and count_bytes params to dd in coreutils to help. The following should work though:
#!/bin/sh
bs=100000
infile=$1
skip=$2
length=$3
(
dd bs=1 skip=$skip count=0
dd bs=$bs count=$(($length / $bs))
dd bs=$(($length % $bs)) count=1
) < "$infile"
