A little more on the default keyboard layout:
Alt and Sym seems to work identical as modifiers.
Sym+S gives the 'Beta'.
Sym+E followed by the base letter gives á, ć, é ...etc.
Sym+U followed by the base letter gives ä, ë, ...etc.
Sym+I followed by the base letter gives â, ê, ...etc.
Sym+N followed by the base letter gives ã, ñ, ...etc.
Sym in combination with `(left of Q) gives à, è, ...etc
Sym+C gives ç
So if that is enough for your usage, you are in luck. 🙂