This is weird. I can have a lookup field which references a form in another creator app... but cant have descending order within the lookup field. I looked at the syntax for how the form lookup is declared and figured I'd try and manually build the lookup field contents
mylist = othercreatorapp.formname[formfield == input.myfield] ;
The above errors with Form 'formname' does not exist in this app! Whereas the form definition has
type = picklist
values = othercreatorapp.formname[formfield == input.myfield].ID
which works OK, aside from not offering descending sort order.