Well, I can't tell you "doesn't work". I can just tell you that I can't see how you could make it work cleanly. The max bubble size doesn't seem to allow an expression. You can type one in, but if you do, it appears to treat it as null and gives you the smallest possible bubble size.
So I think your "reference bubble" may be the best approach, even if it's not perfect. If you're using static min and max, it's easy to make disappear by just moving it off of your chart. Otherwise, set the background color for your X dimension like if(this is my reference bubble, my background color). It's the biggest bubble, so it should be in back. Set it to a size of 400 pixels, since that seems to be the largest allowed size (200 pt, which appears to refer to radius). Don't display anything larger than that, or it'll break your fixed sizes, shrinking everything down. Anyway, the reference bubble is still on the list, and you can still highlight it, and if you're hovering over it and have pop ups, it will pop up. So it's a little annoying. But it seems to work.
Ah, wait, slight problem. The value seems to be used as the area of a square that could enclose the bubble. If that's what's going on, it's easy enough to fix. Just square the value, and then use dual() if you need to see the right value in the pop up.
See attached, though I suspect it's about what you were already thinking.
