Thanks Octavio!
Your method worked just fine, the issue was trying to call the element by element id instead of name:
//works
$('select[name="somecombo"]').append(new Option("option 1", "1"));
//works
$('select[name="somecombo"]').html("<option value='1'>Some oranges</option><option value='2'>More Oranges</option><option value='3'>Even more oranges</option>")
//does not work
$('#somescreenid-somecombo').html("<option value='3'>Some oranges2</option><option value='2'>More Oranges2</option><option value='3'>Even more oranges2</option>")
While you can use JQuery #screenid-elementid for retrieving values, it seems you have to use the name= instead when inserting. Any idea why that would be the case?
Thanks!