[Agama]

This works in a web browser so may work as a template for calibre.

You need to extend the ronu list as far as needed, and set romp to the number of roman numeral pages at the start of the book.

<p id="pagenum" style="text-align:center;"></p>
<script>
  var ronu = [ "-", "i", "ii", "iii", "iv", "v", "vi", "vii", "viii" ]
  var valu = ""
  var romp = 6
  if (_PAGENUM_ > romp)
    valu = _PAGENUM_ - romp
  else
    valu = ronu[_PAGENUM_]
document.getElementById("pagenum").innerHTML = valu
</script>

If you make ronu as big as you think you'll ever need, (e.g. 20 roman numerals), then you just need to change romp for each book.