So - the Boss OC-5 does this job quite nicely on the Lowest Note setting. I mean, it doesn’t have “attack” like a bass would, and the wet only signal sounds pretty muddy without some kind of guitar tone over the top but it does beef up that lower end accurately if you’re chugging chords (and can pull out upper register stuff without an octave effect too).
You can split the signal too - so 2 outputs mean you can pull the octave tone out to a different output. With that in mind I’ve picked up a cheap (but highly rated…) Berhringer bass preamp (BDI21) off EBay, to run the octave signal output to (and then mixing the DI output from that with clean output from the octave pedal going through guitar preamp) - It seems to work / boost the “bass-likeness” of the octave output. Need to try this out in a band setting, but it might help mitigate not being able to find a bassist (apologies to bassists across the world) with some rearrangements of songs