I guess if --plus is supposed to cover all of bash's parameter
expansion capabilites, these should work also:
larryp-MBP:parallel larry$ x=abcabcdefdef; echo $x ${x/#abc/ABC}
abcabcdefdef ABCabcdefdef
larryp-MBP:parallel larry$ x=abcabcdefdef; echo $x ${x/%def/DEF}
abcabcdefdef abcabcdefDEF
larryp-MBP:parallel larry$ echo abcabcdefdef | parallel --plu echo {} {/#abc/ABC}
abcabcdefdef abcabcdefdef
larryp-MBP:parallel larry$ echo abcabcdefdef | parallel --plu echo {} {/%def/DEF}
abcabcdefdef abcabcdefdef