Hi, Will,
Thank you for that.

Here's what I currently do for measuring (99.5% of the credit to
@egreg). In this case, I have a logo image, 10 points of space, and a
rule to fill the rest of the measure horizontally:

	\usepackage{xparse}
	\ExplSyntaxOn
	\NewDocumentCommand{\WidthOfCoffin}{m}{ \box_wd:N #1 }
	\NewDocumentCommand{\HeightOfCoffin}{m}{ \box_ht:N #1 }
	\NewDocumentCommand{\DepthOfCoffin}{m}{ \box_dp:N #1 }
	\ExplSyntaxOff

	\NewCoffin \ruleh
	\newlength{\worki}
	\setlength{\worki}{\textwidth}
	\addtolength{\worki}{-10pt}
	\addtolength{\worki}{-\WidthOfCoffin{\logo}}
	\SetHorizontalCoffin \ruleh {\color{blue}\rule{\worki}{1pc}}

Of course, I could have done this by simply measuring the image in the
\logo coffin, or even prescribing its width by other means
...[width=1in]..., but doing all my copyfitting and layout with coffins
makes for nice, uniform code (and therefore, I suspect, easier to
maintain --- just following the layout of the elementare typographie
cover page made my brain hurt! [No criticism intended other than of my
own limitations!]).

Cheers,
Brent