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Or, as mentioned a II7/9 chord with a 3rd in the bass.Well, I wouldn't say "simply", but I'd say that's probably why it's there. I mean obviously there's also some nice voice-leading between I and bVm7b5. But then you can construct nice voice-leading between just about any two random chords. And actually you could see bVIm7b5 as the normal vi chord with the major 6th in the bass. Not really a big leap.
Well this where things can get interesting IMO. We’re taking about function, yeah? And using comparable substitutions? And that a bv7b5 is comparable to a vi6 chord or a II7? So in the key of F, we have Bmin7b5, Dmin6 and G9 all being of a similar nature? Here’s the rub, I hear those all as consonant to Fmaj, which is a very different relationship than your standard ii chord.Yes, the melody is crucial. But harmonic analysis generally relates to following chords, not to previous ones. At least that's my understanding. Sometimes the preceding chord makes good sense of a chord too, but (IMO) more in terms of voice-leading than function.
What I mean by consonant is, play an Fmaj triad on a piano with the left hand (heck, extend it to the maj7th if you want) and play a Dmin6 with the right hand and it’s consonant. I mean it doesn’t need to resolve. It may not be the prettiest thing but it’s ok, it can hang tight. Same with G9 over F. Now let’s take the usual ii7b5 in relationship to F and make the same substitutions, giving us Gmin7b5, Bbmin6, and Eb9. And try the same experiment with an Fmaj7 in the left and a Bbmin6 in the right, it doesn’t work. Those notes clash, they want to resolve to the Fmaj7. Same with Eb9 on top. And same with Gmin7b5 on top, because it’s a proper ii chord, that’s its job!
Now, I’m not going to tell you a Bmin7b5 over an F triad is consonant, I’m still trying to figure that one out. But if you look at the notes in relation to F it really should be, I think it’s just because of the voicing and built in tri-tone that make it rub and want to resolve.
Anyway, you can probably sum this all up to some kind of modal interchange or lydianization of the tonic (I think I made that word up) or some other term I don’t really understand. But either way I feel pretty confident in saying I don’t really see a bv7b5 as being a common ii7b5 move.