Excellent idea. Here are the results from Kindle and Nexus 7 with default.monospace & textsize=12.
On Kindle sp=11, i=11, W=11, ─=18, │=18
On Nexus sp=10,i=10, W=10, ─=10, │=10
Using the external font unispace.ttf the results are:
On Kindle sp=14, i=14, W=14, ─=13, │=13
On Nexus sp=12,i=12, W=12, ─=10, │=10
So this explains why when I swap in the box characters on Kindle things get messed up.
On Kindle the box characters are much wider with monospace and a little smaller with the external font.
The external font is also not monofont on the Nexus with the box characters being a little smaller.
Anyway, I have my test now. If the standard characters and box characters are not the same size then skip swapping in the box characters.