combining MetronomeMark and TextSpanner - creating new interfaces

[andersvi]

Hello ponders.

Id like to set up a TempoSpanner, using all current settings for
MetronomeMark (offsets, paddings, fonts etc) and placing the
MetronomeMark in the usual manner, but also including a
spanner-interface to get stipled lines etc. towards the next \tempo mark
(or an explicit \tempoSpanStop).

ie.

   rit...........Freeze
   accel..........Mach-II

I know i can set up function based on TextSpanner and manually loading
it with overrides according to the settings for one MetronomeMark, but
this is clumsy and errorprone, needs separate adjustments and doesnt
reflect in midi-playback.

So, how to combine MetronomeMark with TextSpanner in a TempoSpanner?

Thanks.

-anders