It will be nice but I don’t know if it’s possible, last time I checked . Let me explain you how I did.
I open another shell in a split below with tmux then I run fzf in this terminal with my elixir history as input the output (selected line) is written in a fifo. The fifo is read and the line is send as key to the shell running iex.

mkfifo -m o+w $fifo

tmux split-window "iex-history | fzf -s > $fifo"
tmux send-keys -t $current_pane -l "$(cat $fifo)"

Thanks to @dimitarvp who bumped my thread I revisited the project today.
One thing that annoy me is when I typed many line in iex pressing arrow up only show the last line.
I added the fzf option to select multiples lines.
Check the new demo on asciinema

Can this be used without tmux? I am interested in having better history search but I’d prefer such a tool to be mostly independent of the shell it’s used in.