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Yup, it can be done, and you describe how in your last paragraph:
"My thought was maybe there is some way I could insert hidden page markers into the text to indicate where each new page of the original edition begins, and then somehow pull those into the footer text."
That's exactly it. Type the numbers of the original edition in the main text, corresponding to the page turns in the original edition. Hide them by applying a special character style to them that is white (or has no colour), and is a small point size and has a horizontal scale of 1% -- so small that they are unnoticeable in the flow of the main text.
Then, create 2 "running header" text variables based on that character style. One should be set to find the first instance on the page, the other the last instance on the page.
Then insert those text variables on the master page, and it will all work automatically from that point on.
More info about text variables: Create and edit text variables in InDesign
That worked great! Thanks so much, you are a genius! I was not aware of the running header functionality of tracking a paragraph or character style, but it was easy to implement and works quite well.
The only issue I had is that I could not do proper page ranges. Most or nearly all of the pages contain type from one or more pages of the original. But usually the starting page marker comes from the preceding page. With this technique, ID will always take the first running header defined on the current page, instead of the one preceding the first text on the current page (which would be from the previous page). So instead of showing that the page contains text from pages "15–16", it will show "16–16". To work around this, I had to discard the idea of showing a page range, and just put the first matching page number. Still, it works pretty well.
Right, I kind of ignored that complexity in my answer.
As you say, I don't think the page-range thing can really be done, so your compromise of only displaying a single page number is probably a necessary one given the available tools...
I think Gabe Harb's Power Headers add-on (Power Headers | in-tools.com ) could do this. But I'm not sure it works with the latest versions of InDesign. The highest number I'm seeing is InDesign 2015.
Hmm! … If I understand well …
In the new edition, page 15, you begin an old edition text we originally found, in this old edition, page 24 and this text continued until page 29.
In the new edition, this old text beginning page 15 finishes page 18.
In the new edition, page 20, you begin an old edition text we originally found, in this old edition, page 38.
In the new edition, this old text beginning page 20 finishes page 21.
So … what you want is something like this (on the new edition pages):
page 14: 14
page 15: 15 [24-29]
page 16: 16 [24-29]
page 17: 17 [24-29]
page 18: 18 [24-29]
page 19: 19
page 20: 20 
page 21: 21 
page 22: 22
Michel, for FRIdNGE
Here is an actual example from the first chapter.
New edition pages Old edition pages 5 9 6 9–11 7 11–12 8 12–13 9 13–14 10 14–15 11 15
So you'll see that the new edition is more dense than the original, by a ratio of (very roughly) 1:1.25 or so. (The first page of each chapter is less dense, because of how we handled chapter titles.) So while there is not a huge difference in the amount of copy per page, it gradually adds up. Most pages in the book contain copy from parts of two pages in the original (or as few as one, or as many as three), but with significant overlap from the pages before and after.
You don't really answer to my question! …
What is the result on the presentation of the page numbering in your sample?
Page 5: 5 
Page 6: 6 [9-11]
Page 7: 7 [11-12]
Page 8: 8 [12-13]
Page 9: 9 [13-14]
Page 10: 10 [14-15]
Page 11: 11 
Could you complete, correct and maybe show screenshots of what you try to get?!
At the top of each page, near the outside edge, is the new edition page number (standard page number).
At the bottom of each page, near the outside edge, in brackets, is the original edition page number(s).