Xavier Scheuer requested a new feature here:

http://lists.gnu.org/archive/html/bug-lilypond/2013-02/msg00056.html

as follows:

It would be great to have a TempoTextSpanner, for example in order to handle common notation such as

rit. - - - - a tempo or rall - - - -

I do not see this request tracked yet. Bug squad, could you add it? Maybe Kieren or Mike could give a proper description. Link to the original thread: http://lists.gnu.org/archive/html/lilypond-user/2013-02/msg00064.html

Later in that thread Jay Anderson clarified the feature request:

http://lists.gnu.org/archive/html/bug-lilypond/2013-02/msg00058.html

as follows:

If I'm understanding the request correctly this functionality isn't currently available. Text spanners live in the staff. TempoTextSpanners would live in the score and be set like current tempo marks (bold and above the system). I've often wanted this feature as well.

I'd like to vote for this feature, too.

The power of text spanner is the customizability of their bound details.

The power of marks is their alignment capabilities.

It would be really wonderful to combine this functionality.

One of the mailing threads above asks for a syntax proposal. I think it’s good to keep this in line with current \tempo indications, that is, no postfix notation but an event on its own which comes before the notes. Personally I’d prefer a threefold distinction here: – \tempo for absolute tempo indications (as is). – \agogicMark for relative tempo indications like "rit." or "broader", to be printed in medium italic letters. – \agogicSpanner for relative tempo indications spanning some time like "rit. - - -" or even "al -- lar -- gan -- do".

Pseudo code:

{ \tempo "Allegro" 4 \agogicMark "accel." 4 4 4 \agogicSpanner "rit." 4 4 4 4 \tempo "Poco più lento" 1 }

roughly equivalent (in a single-staff system) to

{ \tempo "Allegro" 4 4^\markup \italic "accel." 4 4 4-\tweak bound-details.left.text "rit." \startTextSpan 4 4 4 \tempo "Poco più lento" 1\stopTextSpan }

– the difference being that \agogicMark and \agogicSpanner would be collected above the topmost staff, and \tempo and \agogicMark would automatically end \agogicSpanner (perhaps it should be possible to turn that off, though).

I wanted to add that the \agogicMark suggestion is on another leaf of course. It’s an idea I had yesterday, but implementing \agogicSpanner (or \tempoSpanner) would be independent of that.