Determine \voiceXXX implicitly

[ul]

Hi,

I recall that we had such a discussion some years ago when we worked at 
the Layout Control Options for Frescobaldi. IIRC there wasn't a viable 
solution, but maybe things have changed and maybe I know more today ...

Is it possible to determine from inspecting a grob whether its 
properties have been set automatically or by explicit voice attribution? 
Concretely I'm talking about attribution through the << { } \\ { } >> 
construct.

When dealing with a Tie or Slur object I can see its 'direction 
property, but I can't see if that has been calculated automatically, 
manually overridden through \override or \tweak, or set through a 
\voiceXXX command or the polyphony construct.

But *is* there a way to determine such a situation? I think the 
\voiceXXX commands only set a number of properties, but there's no 
information about that "act", right? What I could imagine is adding a 
custom grob property and set this by redefining \voiceXXX. But can I do 
something comparable for the temporary polyphony construct?

I need this because in my current project I need to place ties not 
according to LilyPond's algorithms but always opposite of the stem 
direction - but only in a \oneVoice context. With \voiceOne also ties 
have to go upwards, just like usual.

Any suggestions?
TIA
Urs