Ok this was teasing me too much - I had to go ahead, find an RTX 4090, and whip out some quick code. 48 leading zero bits:

000000000000ff6b402cb2c12d4d5ec33b2d0d3ad8982835ede2dfeb669fc705 "Bonjour from mrb"

CUDA is nice. I had only developped for AMD previously. For the record the nonce+data that hashes to the above SHA256 is "000000000000000000003c81587599edBonjour from mrb"