Currently, I'm thinking of an easy hack in the event.pm file:

use vars qw(%keybinding);

%keybinding = (
  ... default keybinding
);

my @user_bindings = do 'user_keybinding.pl';
while (@user_bindings) {
  my ($k,$v) = splice @user_bindings, 0, 2;
  $keybinding{$k} = $v;
};

sub key_down_filter    {
  my ($sci_frame, $event) = @_;
  my $key = $event->GetKeyCode;
  if ($event->AltDown) {
    $key = "a$key";
  };
  # same for shift, ctrl, meta, footpad, escape

  if (exists $keybinding{$key}) {
    # dispatch
    $keybinding{$key}->($sci_frame,$event,$key);
  };
};
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Update: I should add that using this scheme, it's not really easy to do multi-key combinations a la WordStar or vi - to implement these, multiple dispatch tables are needed. But as a first start I think this kind of keybinding will be enough.