In the current environment, "inline" formatting is just not possible. The output of a meter is not some stream of characters rendered a character at a time as it is displayed, but really a graphical "object" built all at once and displayed more or less as an image.
It would take an entirely different approach to how Rainmeter works to accomplish HTML/CSS type formatting. You never know though..
For variable lenght text you might use a fixed width font and measure the legnth with a lua script. This becomes more of hassle.
Maybe if it's possible, having some sort of auto-resizing of string meters based on actual (pixel) size of the text would help work around this limitation.