You can't, but you can use a callback to achieve the same effect.
$ perl -MEncode -E'say encode("iso-8859-1", "ab\x{2660}d\x{E9}f")' \
| iconv -f iso-8859-1
ab?déf
$ perl -MEncode -E'say encode("iso-8859-1", "ab\x{2660}d\x{E9}f", sub { "*" })' \
| iconv -f iso-8859-1
ab*déf