So I currently have a ScrolledPanel that contains a number of TextCtrls that are placed in a vertical BoxSizer programmatically. The reason I'm doing this instead of just appending lines to one big scrolled TextCtrl is so that I can also add other controls in between the TextCtrl, such as images or stylized expand/contract folding stuff.
However, this particular implementation is causing a problem - namely that it is impossible for the user to select text across multiple TextCtrls. Is there a way to do this that will be fast, clean, idiomatic, and not especially kludgy? Is my best bet to write a pointer-location text selection algorithm that essentially reinvents the wheel for the text selection stuff of the underlying native libraries, or is there an easier way to embed other controls inside a multiline scrollable TextCtrl, or even select text across multiple TextCtrls natively?
I would stay away from trying to reimplement text selection controls if at all possible, since that is bound to turn very messy very fast. Another way you could tackle this issue would be to use a single multi-line textctrl widget with the other widgets tacked on over it. This is also messy, but less so. 
You can place the other widgets over the textctrl simply by placing them directly over the same position as the textctrl, so long as the other widgets have the same parent as the textctrl. This should work, so long as you don't overlap with the vscrollbar (or, better yet, remove it entirely with style=wx.TE_NO_VSCROLLBAR).
The next thing you'll need to do is pre-fill and space your textctrl so that the user has control of text only right after the position of each widget. You should have each line of text with a different spacing setting, set with the spacing options of wx.TextAttr (the more generic versions of double-spacing, etc), which you calculate based on the particular widget spacing you've given your app. This is necessary to force the user to type only exactly where you want them to. 
Next, you'll need to set up a binding to the textctrl newline character that recalculates the spacing needed for each line. Once you've figured out how to handle spacing, this shouldn't be too difficult. 
Finally, after you select the text, just reset everything to the same spacing, or whatever else suits your fancy, so that you don't get awkward linebreaks when you paste it back in elsewhere. 
I know this is a complicated answer, but it's a complicated issue you raised. This is, I believe, the most efficient way to solve it, and avoids all the bugs that would arise from completely overhauling the textctrl, but it does involve messing around with auto-correcting linebreaks and spacings, which can be a little tricky at first.
Select text over multiple TextCtrls in wxpython