Got it to work!

But I did it the other way round now. I changed and compiled a new kernel. Now all the RDS datas are streamed to two independent devices (/dev/rds0 and /dev/rds1)
First I hoped I can do it without to modify the kernel. But now I think it was the easiest solution.

Disadvantage: Everytime time Mark Lord is releasing a new kernel I have to rebuild my own kernel. :-)

RDS data is now flushing trough /dev/rds1 device while the player is running!
Hope to release a preview version of my TMC software soon!