Can you reference a page number as a roman numeral if it is in the introduction and only roman numerals are given and not page numbers.
AGLC3 seems to provide no guidance on whether Roman numerals can be used as a pinpoint (page) reference in the way that you have, although the alternative of substituting '20' for 'xx' in your footnote would seem to be misleading.
AGLC3 1.1.5 Pinpoint References:
A pinpoint reference to a page should appear as a number...
AGLC3 1.4.2 Above and Below:
You seem to have complied with the citation rule, unless referring to one of the excluded resource types that are nominated there.