I just did a search in the FR for "Schubumluftventil" and found 2 modules, LDUVST and DLDUV they say nothing I can see about anything having to do with overboost. I also tried just "Umluftventil" but it brought me to every spot the first search did through all the pages. I was thinking maybe the definition would be in some other module and the search would find it but there wasn't anything else. This leads me to believe the N249/DV doesn't have a function for overboost.
I also tried searching doc for B_ldsua, B_ldsuas, B_ldsuad AND I identified variables in disassembly and search for all writes. I found the same as you. Hence, why I brought this up.
Thinking about this for a sec I don't want the DV to dump while the turbo is overboosting because taking the load off the compressor wheel I believe would make for an over spin condition without the N75/WG being dumped also. I could be wrong.
Very good point.