Built-in versus embedded fonts

As an outgrowth of the "Helvetica" reference (and I've got working code 
to avoid it now, but would like the Canvas parameter for 
initial_fontname in time, thanks), I found some code that is using 
"Times-Roman" (also built in) to determine which string is longer... The 
strings are _actually_ placed in the document using "Times New Roman" 
(TTF)... but...

1) pdfmetrics.stringWidth doesn't seem to work for TTF fonts.
2) I couldn't find an obvious way to obtain the information for TTF fonts.
3) If kerning gets implemented, the stringWidth should reflect that.

I vaguely recall implementing this "proxy for reality" late one night in 
a coding frenzy... is there an easy way to obtain stringWidth for a TTF 
font?