It actually is a lot more complicated than horizontal rules.

I'm an avid user of paragraph rules, and though I've managed to come up with some pretty unorthodox stuff before (see my Paragraph Rules tutorials soon on Unzipped and The FontFeed) there is to my knowledge no way to achieve vertical rules that match the height of paragraphs.

The easiest way to make a vertical rule is to define a very short, very heavy rule: a 1pt wide 24pt thick horizontal rule is -- indeed -- a vertical 1pt rule that is 24pt long. To make the rule only 1pt wide, specify an indent from right that is 1pt less than the column width. Elementary my dear Watson.

Now the main problem is that Paragraph Rules are attached to the paragraphs -- you can position one in relation to the first line of the paragraph and one in relation to the last line. How many lines the paragraph counts has no effect at all. So even so it's possible to specify vertical rules next to both the first and the last line in the paragraph, it's impossible to specify a vertical rule next to all the other lines. The only way to do so would be to convert every line of text to a single paragraph by using hard returns, but this completely negates what you want to achieve, as you explained text flow isn't finalised.

So I'm really sorry, but to my knowledge what you want to do is impossible. :^/

Hey Ben (& Yves),

Your solution makes sense and won't be too hard to implement. Of course, I have a bunch of headings, subheads, etc., to deal with, all of which I'll want to mask the rules you suggested, but that's a small matter. I suppose, as Yves mentioned, the trick will be to make sure that my "mask" rules are exactly the height of the styles they're added to, so that I don't have tiny bits of vertical rule showing through. I'll also likely move the vertical rules slightly inside the text frame, as I've had vector objects shift ever so slightly during RIP at the printer.

Thanks for the suggestion, and hopefully a simpler method will be introduced in future InDesign releases...

Thankyou Yves!

Ah yes I didn't allow for headings and subheads.

It occurs to me that these could be dealt with on a separate layer, perhaps with white text box backgrounds for the masking or as a massively baseline-shifted part of the first line of the paragraph style if you want to keep everything on one layer.. ie. if your leading is 15pt and the white para rule is masking 45pts of inter-para spacing (three lines), then a subhead can be baseline shifted another lines' worth, 15pt upwards from the first line of para to appear in front of that masked 45pts of space.

You'll need to experiment with this...

Re: ben_archer's tip about putting the vertical rule(s) on the Master page and masking it out with Paragraph Rules

Excellent workaround, but I found that using the Underscore Option in Paragraph Styles worked even better because - once the weight and offset were set effectively - it applied to paragraphs of any number of lines.