One solution would be to get all available options and the already selected ones then find out if they match and create a select element with a selected parameter set to it.
I don't think this can be done using the Form helper
(I know it can be done with just a single value, but have not tried it with multiple values)
Example
Get all the available options (try to get something to uniquely identify them like the id)
$all_options=Model::lists('id','name');
I'm using the lists method that will return me an array with all the available id's as the value and the name as the keys of the array.
Then get all the selected values
$already_selected_options =Model::with('related_model')->lists('id','name');
(If you are using eloquent something like this to get the related ids will work)
Then in your view
<select multiple>
@foreach($all_options as $name => $id)
@if(in_array($id,$already_selected_options))
<option value="{{$id}}" selected>{{$name}}</option>
@else
<option value="{{$id}}">{{$name}}</option>
@endif
@endforeach
<select>