Drat. While Barb was providing her response, I was experimenting with a similar answer:
I gotcha.
I'm going to make the assumption that you know how to create page number markers already to automate part of this process. If not, check out the link below:
Creating page number markers on Master Pages
This was a fun thing to figure out. Try this and see if it works for you ...
Go to your left Master Page and draw a vertical line outside the page margins as shown at right by getting your Line tool,
dragging straight down holding the Shift key to create a perfect vertical line.
Get your Type on a Path tool. You do that by placing your cursor on the lower-right corner of your regular line tool, where you see the downward-pointing triangle, as shown in the illustration immediately at left.
Click on the line with the tool, as shown in the image below. Place a current page number marker with the Type>Insert Special Character>Markers>Current Page Number. On your Master Page, it'll appear as aletter corresponding to the page shown below-left, then make the line "disappear" by clicking on it with the Arrow/Selection tool, giving it a stroke of 0 pt as shown below.

Now's where the fun came in. Get your Text tool, 
highlight the letter again and go to the Type>Type on a Path>Options... menu command. In the panel select the Stair Step option in the Effect: options box, then select OK.
This is where fitting the page number type gets challenging. When you go to, say, page 222, the copy looks like it's 
crashed vertically, like the example at right. Working with normal type-fitting tools within InDesign like kerning, tracking and leading didn't change anything except the alignment on the top 2 numbers from left to right.
What finally did work, to some degree, was to effectively double the type size and select
the
Small Caps button in the Control Panel, as shown at right. Doing that resulted in the page numbers stacked with just a little bit of space between the numbers like the illustration shown below:

Once you get the result you want, go back to your master pages, copy the marker and place it to patch on the right-hand page. Then admire your handiwork.
Try this and see how it works for you. And thank you for the interesting question. Normally folks providing answers get to share things they already know. Finding your answer was a fun experiment for me to discover something I really never knew before.
Good Luck,
Randy