$did().selstart works. Using $mid, $did().selstart and $did().selend it should be easy to insert text into an editbox at the cursor position. Example:
Code:
alias inserteditbox {
  var %dname = $$1
  var %id = $$2
  var %itext = $$3-
  var %text = $did(%dname,%id)
  if ($did(%dname,%id).selstart == 0) { did -o %dname %id 1 %itext $+ %text }
  else { did -o %dname %id 1 $mid(%text,1,$did(%dname,%id).selstart) $+ %itext $+ $mid(%text,$calc($did(%dname,%id).selend + 1)) }
}