[luatex] How can I display arbitrary UNICODE characters?

[Ana Lúcia de Moura]

I'm currently developing an automatic pdf document generator using LuaLatex.
Unfortunately, the documents can contain arbitrary user-generated
utf-8 text which results in "blank" output if the corresponding glyphs
are not present in the selected font. Is there a (reasonable) way to
avoid this?

Ideally, I would love to to be able to define a chain of fallback fonts.

If that is not possible, another thing that would work for me would be
to be able to associate character ranges to specific fonts. (eg. a
font for the range of Chinese characters, another for Greek, ...).
It is worth reminding that I have to deal with arbitrary text (i.e,
Chinese names inside a Portuguese text) and font switching would need
to happen on a char-by-char basis.

Finally, if none of the above are possible, at least I would like to
be able to produce a "char not supported" glyph or fallback to system
fonts, instead of producing blank output.

Thanks in advance!
Ana