@adamhsparks well, it might have been a bit of cheating, but besides the lines of config:
[keys.normal]
C-c = { C-c = ["goto_prev_paragraph", "collapse_selection", "goto_next_paragraph", ":pipe-to ~/.local/bin/send-to-tmux", "collapse_selection"], c = ["toggle_comments"] }
[keys.select]
C-c = { C-c = [":pipe-to ~/.local/bin/send-to-tmux"], c = ["toggle_comments"] }

There are actually two bash scripts involved as well:

~/.local/bin/send-to-tmux.sh
#!/usr/bin/env bash
while IFS= read line; do
        tmux send-keys -t "helix_term" "$line" "Enter"
done

~/.local/bin/helixterm.sh
#!/usr/bin/env bash
# unset $TMUX variable to enforce spawn
export TMUX=""
# Grab command
if [[ -z "$1" ]]; then
    CMD="$SHELL"
else
    CMD="$1"
fi
# Create new tmux session
tmux -2 new-session -s "helix_term" -d "$CMD"
# Attach to tmux session
tmux -2 attach-session -t "helix_term"

I now can simply start my #rstats session with helixterm.sh R in a split terminal next to my script opened in helix and send code to it (both in normal or select mode) with 2x Ctrl-C. Of course that also works on a remote R-session via ssh and whatnot.