Thanks everyone for your responses, it looks like bypass on right channel on left is the winner.
The hypothetical pedal in question is a preamp, though for now it's just a proof of concept. I was contacted last week by a big-name bassist about doing a custom build, and if it turns out as cool as we both think it will then I'll keep it in the lineup as a standard offering. I don't have all the details yet as the pedal only exists on paper, but it's a solid-state preamp with speaker emulation, and the final version will also have a Lightning Boy D.I. output.
I'm starting with just the preamp to make sure everything works and is useful, then anything that isn't needed will get taken off, and anything that's missing will get added. So it will probably be at least two rounds of prototype before a final product. He said he isn't in any hurry, but I've set the personal goal to have it ready to go by Christmas.
So that's where most of my R&D energy will be going for the next couple months.