After getting the suggestion to use the layouts package for visual debugging of lengths, I came up with the following example, which I compile with xelatex test.tex:
\documentclass[12pt,a4paper]{article}
\usepackage[pass]{geometry}
\usepackage{tikz,enumitem}
\usepackage{fix-cm}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{Junicode}
\usepackage{layouts}
\begin{document}
\pdfpagewidth=148mm \pdfpageheight=210mm % for PDF output
\paperwidth=148mm \paperheight=210mm % for TikZ
\begin{tikzpicture}[overlay,remember picture,inner sep=0pt,yscale=-1,outer sep=0pt,anchor=north west]
\begin{scope}[shift={(-\oddsidemargin,-\topmargin-\headheight-\headsep-\marginparsep)}]% layer1
% \path[anchor=north west] (0,0) node[below right] (dbg) {% this goes to (0,0) but after margin
\path[anchor=north west] (current page.north west) node[below right] (dbg) {%
\begin{minipage}{\paperwidth}%
\setlayoutscale{1.0} %
\drawdimensionstrue %
\pagedesign %
\end{minipage}
}%path
;
\end{scope}
\end{tikzpicture}
\end{document}
This is what I get as output (upper/left/right edges of image correlate to corresponding edges of the pdf page):

Notably, all lengths being read are reported as zeroes; I was otherwise hoping that I could force the layouts page diagram to be scaled to one, and overlaid over the current page using a tikz node - but that apparently fails.
Is it possible at all that I get a layouts page diagram overlaid over a page as I intended to? If so, how?
Many thanks in advance for any answers,
Cheers!
\setlayoutscale{0.68}, I get the page edges on layout to scale almost to pdf page edges). I've seen @WillRobertson around this site, so hopefully tex.se will issue a bump - else, by looking at CTAN webpage, I should email to submit a bug. Many thanks again - cheers! – sdaau May 13 '12 at 11:39