Biblatex: small caps for pages in roman numerals

Here's how you can get Roman page numbers in biblatex to appear in small caps.

First we define a macro mknormrangescromannums that detects if a given number is a Roman numeral and applies additional formatting in that case. The test for Roman numerals is based on the other numeral tests of biblatex (ifinteger, ifnumeral, ifnumerals, ...).

Then we use the <itempostpro> argument of mknormrange (introduced in biblatex 3.13 [2019-08-17]) to call mknormrangescromannums on all page numbers.

documentclass[british]{article}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{babel}
usepackage{csquotes}

usepackage[style=authoryear, backend=biber]{biblatex}

makeatletter
defblx@hook@ifromannum{%
  defdo##1{uccode`##1=`%}%
  doidovdoxdoldocdoddom
  doIdoVdoXdoLdoCdoDdoM}

protecteddefblx@imc@ifromannumeral{%
  blx@ifnumblx@hook@ifromannum}

blx@regimcs{ifromannumeral}
makeatother

newrobustcmd*{smallcapsromannums}[1]{%
  ifromannumeral{#1}
    {textsc{lowercase{#1}}}
    {#1}}

newcommand*{mknormrangescromannums}{mknormrange[][smallcapsromannums]}

DeclareFieldFormat{postnote}{mkpageprefix[pagination][mknormrangescromannums]{#1}}
DeclareFieldFormat{pages}{mkpageprefix[bookpagination][mknormrangescromannums]{#1}}

begin{filecontents}{jobname.bib}
@InBook{id,
  author    = {Surname, Name},
  title     = {Title},
  booktitle = {Book Title},
  date      = {2000},
  pages     = {x-cx},
}
end{filecontents}
addbibresource{jobname.bib}

begin{document}
autocite[xxi]{id}

autocite[12-45]{id}

autocite[hello]{id}

printbibliography
end{document}