New member - Bar numbers placement problem

[Les Editions Valmajour]

Hello everyone! I am new on the list, from France and professionnal 
musician, engraving music as a hobby. Lilypond is fantastic, but so 
powerful that I think it's for now too powerful for my little knowledge, 
and I need your help.

I would like to position the bar numbers for a piano system in front of 
and centered on StartSystem Brace. Like if it was in the place of the 
Instrument Name (or Short Instrument Name).
I've tried to use the invisible Dynamics staff and force the display of 
bar numbers, but the result is not working fine, as you can see in the 
attached screen capture; if there is no dynamics in the system, the 
number is not displayed at the same position

Do anyone have an idea how to do this right?
Thank you so much,
Pierre

Screenshot - 18-10-2014 - 17:07:06.png
Description: PNG image