Hi Ivan,
I did have a short look at the existing "Create wikitext link CTRL-L" button with "auto completion". It should be simple to implement, but we would need to change the UI a little bit. .... And there is no time left to get it into TW V5.1.23 ...
So it seems to be much easier to crate a completely new function as a plugin based on the existing function.
The CTRL-L function allows to select some text and then insert a link, using the text as "pretty link". ... For transclusions, this text doesn't make sense, so the question is, what should we do with the text.
a) Replace it
b) Insert the transclusion after the text and leave the text unchanged.
I would go for b). .. Just some thoughts!!
-mario