"I will spend some more time on this: I'll let you know if that proves fruitful."

After some delving into the source code for Tkx.pm and Tcl.pm, I've now got this working pretty much the way I wanted. Here's an extract of the module code:

...
use 5.026;
...
use constant {
    ...
    ERR_CALLBACK_NOT_CODEREF    => 'Callback is not a coderef.',
};
...
use Scalar::Util 'reftype';
...
use Tkx;
...
{
    use constant BINDPLUS_REF_BASE => 'bindplus_ref_base_';

    my ($tcl_interp, $tcl_sub_ref_id);
    BEGIN {
        $tcl_interp = Tkx::i::interp();
        $tcl_sub_ref_id = 0;
    }

    sub bindplus {
        my ($tag, $seq, $callback) = @_;

        my $callback_ref = reftype $callback;

        if ($callback_ref eq 'CODE') {
            Tkx::bind($tag, $seq, '+' . $tcl_interp->create_tcl_sub(
                $callback, undef, undef, BINDPLUS_REF_BASE . $tcl_sub_
+ref_id++
            ));
        }
        elsif ($callback_ref eq 'ARRAY') {
            if (reftype $callback->[0] eq 'CODE') {
                Tkx::bind($tag, $seq, '+' . $tcl_interp->create_tcl_su
+b(
                    sub { $callback->[0]->($callback->@[1 .. $callback
+->$#*]) },
                    undef, undef, BINDPLUS_REF_BASE . $tcl_sub_ref_id+
++
                ));
            }
            else {
                croak ERR_CALLBACK_NOT_CODEREF;
            }
        }
        else {
            croak ERR_CALLBACK_NOT_CODEREF;
        }
    }
}
[download]

The trick to appending multiple times, was to use a unique identifier for the fourth create_tcl_sub() argument:

$callback, undef, undef, BINDPLUS_REF_BASE . $tcl_sub_ref_id++
[download]

In my tests, I created an initial binding in the normal way (with Tkx::bind()); I then appended a further eleven bindings using the function above. That was nine tests (one for each of the callback forms shown below) plus two that were somewhat more involved (using tk_messageBox). All were called in the correct order.

This works for callbacks in any of these forms:

        sub { ... }
        \&subname
        $coderef
        [sub { ... }]
        [sub { ... }, @args]
        [\&subname]
        [\&subname, @args]
        [$coderef]
        [$coderef, @args]
[download]

One thing this code doesn't do is handle Tkx::Ev() calls in the @args. That's not a current requirement for me: perhaps I'll look into it at a later date.