The discussion around spot colour largely misses the point as I see it. As a professional graphic artist I think its an important thing to have clearly understood.

Colour support has become my #1 problem with using inkscape at work. Well, that mixed with the fact of the difficulty of getting editable artwork to work the same in illustrator (just for setting up separations for output).

LittleCMS doesn't matter particularly as the answer to the original question from Terry Brown. Printing as SPOT colour, means output separations for named colours individually, not mixed as CMYK, RGB, LAB or anything else.

Example:

Pantone 872C is a gold metallic ink (can't be accurately mixed in other systems).
On spot colour output you want a separation of solid black ink (to make the printing plate), labelled Pantone 872 (for the ink that is applied).
To approximate it on screen you want specific RGB values
To approximate it web-safe you need some other RGB values
To approximate it on desktop printer you need some CMYK values

Nasty workaround:
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I started doing graphics in the beginning of the desktop publishing almost 20 years ago, and there was time like 90-91 where you had to abuse the CMYK separations to get spot separations out of illustrator and freehand:

re-color your artwork before sending to the printer,
so 'Pantone 872C' became cyan (for instance), and
make sure the printer had clear instructions replacing cyan with Pantone 872C

Changing applied colours like that in inkscape is laborious for complex artwork.
Its probably less work than the suggestion in this thread of setting up layers for the various colours, exporting them individually and manually aligining the separations later in other programs.

The problem?
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I think the reason this is not addressed in inkscape, is the way that colors are stored and used.

We can use a palette to apply a color. Later we cannot change the palette - and even if we could, it wouldn't change all instances where that colour is applied.

The established solution:
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Illustrator has an additional concept of swatches - where the colour properties of objects are linked to a swatch object. Swatches can be changed, changing all objects/lines/fills/gradientstops that reference them.

This is extremely useful in complex illustrations where you might have several colours that mean things, but need to adjust them later in the process.

A spot colour needs to be defined like a swatch, so that the one definition can be used identically across many different objects, and later used as a separation.

That also allows artwork to move between colour spaces, as each swatch can have a representation defined in each colour space.

That is the real value that the downloadable profiles and swatch libraries provide -- the ability to match output from different devices eg. screen (RGB), web-safe (RGB), inkjet (CMYK),web offset lithography (CMYK with colour profile) and spot colour reproduction.