Specific large "watermark" time signature

[Urs Liska]

I was asked by someone how much effort it is in LilyPond to create a
time signature similar to the one in this post:
http://www.finalemusic.com/blog/new-features-next-version-finale/ but
additionally behaving like a watermark (i.e. grey and in the background).

I came up with the attached proof-of-concept quickly, but I have one
more question: is it possible to make the time signature automatically
top-align to the topmost staff line of the topmost staff in the system,
or to center it on the StaffGroup? manually editing Y-offset doesn't
seem like a good idea, as I don't have the impression there's an easy
relation that could be set up between font-size and Y-offset.

Experimenting with self-alignment-Y didn't seem to have any useful effect.

TIA
Urs

time-sig-2.ly
Description: Text Data

time-sig-2.png
Description: PNG image