I wrote Gibraltar about that very problem a while ago and they said they didn't have one for that size. Not the end of the world, but means you to position it every time you setup and have to think about it, which I rather not have to do.
Having no memory lock for the bottom tube kind of defeats the whole purpose of the stand, because the stand is made specifically for use on a rack. And, one of the main reasons to use a rack is to ensure that every piece you mount on it will always be in the exact same position every time you set up your kit.
I've toyed with idea of using a memory lock designed to lock the base of tripod stands, but they don't have the notches that prevent rotation like memory locks for drum rack clamps have.
The best workaround I've come up with after owning this stand for years now is clamping to the upper tube instead. I put a memory lock on the upper tube (can't remember off the top of my head if it's 7/8" or 1"), above the stock upper tube memory lock (the one that prevents the top tube from slipping down into 1-1/8" outer downtube). I fly the rack bar for the snare higher up on the vertical rack bar. While this does the trick by making my snare setup quick, the disadvantages are:
1) If I want to change the position/height of my snare, I have to loosen 4 things - the rack clamp holding the the stand, both the "workaround" memory lock and the stock memory lock, and the downtube clamp. Only after doing that can I make an adjustment, and then I have to tighten them all up again. If I was able to clamp to the downtube, I would only need loosen the stock memory lock wingnut and the downtube clamp like the stand was designed to make adjustments.
2) If I want to run a deep snare, because the "workaround" memory lock is now on the upper tube, it limits how low I can slide the top tube down.
3) There's no way, mid-performance, that I can quickly swap out snares if I wanted to, if the snares are of different depths.
4) Having the snare rack bar have to be positioned up higher on the vertical upright means the rack bar is now above the crossbar of my double-pedal. So, I have to feel around under the rack bar if I want to make any adjustments to my pedal that require removal of the pedal crossbar.
I was kind of hoping that enough people like you contacted them over the years and they might have come out with one, but I guess that's not the case. Bummer, because other than that issue, it's a darned nice stand.
Do you have a workaround of your own that you're using?