Aligning a tempo marking with a note

[David Sumbler]

Although many scores use a similar italic font for "rit.", "accel." etc.
as they do for dynamic changes such as "cresc.", I prefer to use an
upright bold font - the same as for other tempo markings such as
"Allegro".

The easiest way for me to do this (at least until now) is to use '\tempo
"rit."' or similar.

However, I have now discovered that this does not work too well when the
mark comes at the beginning of a bar which has a time- and/or a
key-signature, because the marking then comes above the signature.  This
is fine for a normal tempo marking such as "Allegro", but looks wrong
for "rit." or "accel.", which I would prefer to appear above the first
note of the bar.

I have tried
\once \override Score.MetronomeMark.X-offset = #4 \tempo "rit.", but
this is unreliable: it works some of the time, but does not work
correctly at the beginning of a line; also a clef change seems to upset
things.  And since a clef change will not exist in all the parts, I
cannot use different values of MetronomeMark.X-offset to get around this
problem.

Clearly, rather than trying to fake it with horizontal shifting, I need
actually to align the tempo mark with the note itself, but I haven't
managed to work out how to do that.

How can it be done?

David