Unfortunately the BL3 service manual doesn't have detailed schematics so I can't be 100% certain, but it is possible that in line mode, they will turn on with detected audio signal on the Powerlink input as well (some earlier Beolabs do). In that case, you could keep your existing Powerlink cables at the speaker end of the setup, and not need any adapters there.
Anyway, I don't think the extra adapters will deteriorate the sound quality, but cable shielding from interference may be a problem. If you need to use the adapters at the speaker ends as well, I think I would rather install them at the speakers, and use a Powerlink cable from the wall socket instead of RCA.
And definitely consult Steve as well, he might have better ideas.