The XmLabel displays an XmString, so you can construct a string with two segments, one of which has your normal text, and the second of which has the text character that corresponds to the symbol that you need in the font which includes that symbol. With this solution, your problem is reduced to finding a font which is guaranteed to be on the system and which has these symbols.

The second solution is to take advantage of the fact that the XmLabel displays a pixmap. In this scenario, you need to render the text, yourself, into a pixmap, render the symbol into the pixmap, and then display the pixmap. This solution will work on all systems but requires much more work.