You can add a colored frame in the background on any page. You can either make a separate layer and set that to non-printing (probably the best way) or set the frame to non-printing in the Attributes panel.
This will be slightly different in behavior, I suspect, than redefining the [Paper] swatch in several ways. First, you won't see that color if you you use Paper for a fill or stroke, and second, it may behave differently on screen when using some effects. To overcome, or at least work around the first problem, I would define SPOT color swatches for the various colors. The bad news is that you can't alias a spot color to Paper in the ink manager, but you can either delete the colors and repalce with the paper swatch, or not not output the spot plates (there may be some trapping issues with this). Decidedely not perfect, but perhaps better than nothing.
Defining spots may also help with the blending problem.
Thanks Peter, your answer is helpful.
But of course It's a pitty, that we don't have a standard solution for that.
It's a pity we can't do a lot of things, but there's a lot of prioritizing that gets done on adding new features (I have no idea how much effort would be involved in being able to copy the [Paper] swatch and retain it's knockout properties). The number of users who would benefit, the business case for adding that feature for those users, whether other users might find a secondary use, how much time it will take to implement, and what else is also on the wish list all play a part.
If color accuracy or management matters you also have to consider ink transparency. The Paper color assumes a transparent ink when Overprint preview is turned on, so 50% magenta on green paper looks like this:
While using a swatch looks like this—the transparency of an offset ink is not previewed:
You could put the paper simulation ink layer on top and set it to multiply, but in either case you can't expect the preview to be accurate.
Thank you Rob! Your thoughts are useful too.
Of course I don't expect a perfect simulation,
but good approaches are really wellcome.