The Unicode standard includes non-spacing accents, so you can stack 'em as much as you like.  

Working out how to type them conveniently will depend on your keyboard handler.  Linux comes with many options in this area.  Ibus and m17n include ready-made Roman IAST and Devanagari Sanskrit keyboards, but it's very easy to make one's own combinations ad libitum for anything not in the standard keystroke repertoire.   In the control file, you put what you want to type in the left column, and what's meant to show on your screen in the right column.

Finally, what stacked accents will look like depends on the font you select.  Some fonts handle this more elegantly than others.

Best,

Dominik