You mentioned in synth_dc.h "// if only C language provided a way to test Q status bit...."
Here it is :) . That return (out & 0x8000000)>>27; translates to only one more instruction : ubfx    r0, r0, #27, #1 , i decided to use the c -variant because it is better to readable.

I added a function to clear that bit ( clr_q_psr() ), too. Unfortunately, three instructions are needed (read-modify-write)