Well, that was embarrassing. Definitely should have tested outright before posting here. Setting it explicitly worked.
What is interesting is that the method for setting it in a loop didn't work.
What I've done is created a delimited global string in the following format:
"a^39|b^39|c^39|d^41" etc. that includes all characters in my character set. The end of the above list is "Z^44| ^14" indicating that a "space" is 14 pixels wide.
I loop through this string using tokenat on start of layout to set all character widths, but it doesn't seem to register the "space". If I use a manual line after the loop is over to explicitly set the width of space, things work fine.
Anyway, thank you very much, @Ashley, for adding this plugin officially. If you're still taking feature requests for it, could you add a property that allows for adjusting the pixel width of each character based on the character that follows it? I understand if this is a little too convoluted to be worth the time, but it really would allow for non-mono width fonts to be perfectly reproduced with Sprite font.
To give an example of what I mean, take two strings, each two characters wide:
In 48 point, Arial Rounded MT Bold font, the width of the first 'a' in "aa" is 38 pixels. The width of the 'a' in "ab" is 40 pixels. It turns out that regardless of what the first letter is, if the letter 'b' comes second, it ALWAYS results in the preceding character being 2 pixels wider than if an 'a' were to come second. So, what would be perfect is if I could set a baseline width per character and ALSO set a modifier for each character that says "add this amount to the width of the preceding character". The value could be negative, of course.
For now, I've just averaged the width of each character and I'm hoping it will continue to test to make sure it looks ok. We're only talking about a few pixels. For what it's worth, the letter 'j' is kind of the outlier in how much it affects the width of whatever character precedes it. 'f' is the other culprit.
Anyway, thanks again for the plugin. I can now determine the exact width of strings and I'm not dealing with blank text boxes as I tried to trim the whitespace from dynamically created and resized text boxes between Chrome and iPad... talk about maddening :)