Dear Reinhard / Thanh -- Many thanks for your comments. I'm sorry that "adding new features to pdftex is not desired now", but I guess if it's good enough for Don, then it's good enough for Thanh :-) As to Reinhard's closing observation :
And changing the matrix isn't sufficient. You'll certainly have to adapt the .tfm files.
I can't see why this is the case : once PdfTeX has read the TFM file, is it not sufficient "simply" to manipulate its internal version thereof, rather than the TFM file itself ? After all, once it has produced the Pdf file there is no longer any need for access to the TFM, so what does one gain by modifying it ? ** Phil -------- P.S. I've kludged my way around it for now, but it's not pretty; see "Hu'o'ng" in the last line of the attached.