Single preburner turbine as electric generator, electrically coupled to oxidizer/fuel electric turbopumps, would allow shaft decoupling and temporary boost via secondary power sources. If you change your sizing for the preburner such that its generator output is matched to throttledown/maxQ inputs for the turbopumps, then a secondary battery can provide the boost assist at higher throttle settings with more fine tuning. I guess a hybrid electric staged combustion setup, where the preburner provides bulk electric power?
Err, so electric LP oxidizer/fuel turbopumps feed (electric HP oxidizer/fuel turbopumps + preburner power generator), plus the boost battery?