I wouldn't hope of a fretless Combustion, for now at least. I think I even asked Sheldon the same question a year or so ago.
Fretless necks have thicker fingerboards - to accommodate for the fret height. Fretless boards need special care fretted fingerboards do not need. I believe I've asked (a year ago, perhaps) if Sheldon was willing to de-fret a Combustion himself on special order, and his hour is just too expensive. That job could match the price of the bass itself. He could be spending that time making an expensive bass, or developing some new stuff, I don't know.. So it makes sence.
I'm not the man himself and I'm not speaking in his name, but the Combustion is, still, just an offshore bass and we need to remember that. Any changes need a lot of time to develop, and a lot of time to train the builders to do it right. Many sales need to be made in order to justify a change or an option. Good fretless boards require a lot of skill and time, and there just wouldn't be enough demand. Well, not now, maybe in the future.
...I just realized that, while really believing what I've said here, I've also proposed the C2 mod. I'll go to my corner now

There is a ton of reading material in DYI de-fretting on Talkbass. You could just remove the frets, fill the gaps, sand the fretboard, epoxy and lower the nut to compensate for the fret height. The neck would feel thinner compared to a fretted Combustion, but it's a cheap and viable option.
Or, you can get a fretless AB-I or II. I don't think fretless AB-Zs are possible with the maple fingerboard.
And to add, OT: when the Afterburner GL (the Combustion prototype) was first proposed last January I got a job and started saving, hoping to de-fret it. Then it was postponed over and over again, so after a long while I collected enough to buy a fretless AB-II. I'm a happy camper now, just need a C2 to cover the fretted/MM ground.