Altium place string (letter with macron)
You can select any TrueType font you have on your computer and use whatever weird symbols you like. The choices of sans-serif and serif apply only to stroke fonts. Using TT fonts (potentially) sacrifices portability if you need to edit the file on another computer that doesn't have that TT font installed it won't work, and it will make your Gerber files bigger (but that's not really a problem in 2015). 



I don't know of a way to put an overbar over a set of characters like you would on a net label (for example R\E\S\E\T\ for inverted reset).