Mixing notehead sizes in a 'chord'?

[Hans Forbrich]

In the piece I'm working - SATB + Piano - the Tenors have 'alternate'
notes which are designated by a reduced notehead size.  Since IMHO these
are 'one voice', ideally these alternates could be represented (Lily
1.7.13) as

<<c \tiny a \normfontsize >>4.-( <<e \tiny c  \normfontsize >>8-)

However, trying this I find that all noteheads in a chord become the
same size.

A second variant would be to create 2 voices ... (which should also
allow separate slurs/ties for each pair - also needed, but probably a
separate question)

< {c4.-( e8-) } \\ { \stemUp \slurUp \tiny a4.-(  c8-) } >

This seem to work some of the time, but occasionally the stems get out
of alignment.  More accurately, the alignment of the tiny & normal
noteheads is on the 'front' of the grob which, when stems are up, causes
the tiny and the normal stems to both be visible. (Making stems
transparent results in a gap between the tiny notehead & the 'master'
stem, therefore not acceptable.)

Related questions:
1) Any hope of getting mixed notehead sizes in a chord sometime?
1a) If it's there now, how do I do this?  (What did I overlook?)
--- or ---
2) How can I use 'stem-based' notehead alignment or nudge the tiny notes
to line up with the stems of the normal font?

Advice, corrections, pointers to appropriate doc, etc. appreciated.
/Hans