Does it also inhibit reappearance of the FD?
According company info it does:
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The Flight Control Primary Computer (FCPC) has been modified by installing a new software standard; P11AM20A 2K2.
The only improvement introduced with this software update concerns the autopilot inhibition under unreliable airspeed condition.
When there are significant differences between all airspeed sources,
the flight controls revert to alternate law, the autopilot (AP) and AutoThrust (A/THR) automatically disconnect and the Flight Director (FD) bars are automatically removed
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The BUSS, if fitted, must not be used above FL250. The
reason for that is that the back-up stall warning threshold does not change with Mach number and the selected value is too high above FL250.
The FL250 threshold is explained in FCOM as:
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Cases of unreliable speed situation caused by temporary contamination of the pitots with water or ice (typically above FL250), usually disappear after a few minutes.
The flight crew flies pitch/thrust and activates the BUSS only if the unreliable speed situation appears or remains below FL250.
This distinction (FL 250) ensures a good balance between crew workload and safety
(continued flight using the BUSS or temporary use of pitch/thrust tables).
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But you are correct: the Backup Stall Warning values are fixed values for S/F config.