Well, I happen to be on 2nd-shift today - and my wife woke me up at 7am so I had half a day to spend!
And I like figuring things like this out... it keeps my brain active.
As far as being overkill...it's probably a lot easier to do it this way than adding a bunch of nbsp. You can create a clip in Sigil so a click of the mouse will insert the <span> around selected text (or insert empty spans if no text is selected). You could even create a fairly simple regex to automagically add the spans, drastically reducing the number of lines you would have to handle individually. the nbsps would only need to be added
AFTER the text that needs to be spaced in the middle.
Adding the <div> with a max-width would remove the necessity for people to mess with the font-size...and some of your older readers would probably be very thankful for not making them get out the magnifying glass to read that incredibly small font.
And...as a last comment... using pixels to determine the maximum width is probably not the best way to go - for exactly the reason that people could change the font size. If the font-size is too large/small then pixels wouldn't do what you'd want. It's probably better to go with something like EM when determining max-width....or, even better, using media queries to adjust your layout depending on the type/size of the display.
OK, now I have to go to work!
Cheers,