I noticed that there appear to be two different customization variables that purport to control the configuration of the speedbar frame. There's the "obvious" one labeled "Frame Parameters" from the "Customize" option on the right-click menu in the speedbar. There's also one that I've used before, named "speedbar-frame-plist". It appears that changes I make to the obvious one are ignored, but changes to "speedbar-frame-plist" are respected (at least "left", which is the one I'm trying to set).
I'm aware of the fact that these two variables are structured differently (the obvious one uses the "(key . value)" syntax, and the other separates the key and value in the customize UI. I added "(left . 682)" to the obvious one, and it had no effect, whereas adding a Property and Value of "left" and "682" to the other one changed the speedbar frame location. Is this perhaps happening because "speedbar-frame-plist" overrides the value of the other variable?