How do you color match an RGB or CMYK color from a digital photograph,
onto a canvas using a mix of Acrylic or Oil paints?   For example, if
you have the color R:152, G:148, B:235, what process (using readily
available paints and tools) would give you a scientically matched
color?

There might be some large format plotters the print directly on canvas.

You could then match by eye as you brush on you acrylics.

I am only interested in color matching, not necessarily matching the
whole picture.  I was thinking more along the lines of using some type
of measuring device, like a syringe that would give the appropriate
ratio of Red, Green and Blue paint to make the required color match. 
In theory the mix ratio of paint would match the RGB ratios of the
pixel, but what paints would you use?  Are there Red, Green and Blue
paint products that match the Red Green and Blue tones used in the RGB
digital color model?

There was a similar question a few months ago  - that could not be
satisfactorily answered.  But it might help you understand the
problem.

http://answers.google.com/answers/threadview?id=429179

Paints are not mixed the way digital and printed images are created,
which is by "fooling" the eye to "see" a color that is a combination
of pixels in the colors the sight nerves react to.

The links to that question are full of information, but not a formula
for what you want, sorry.