Most likely a two-step (choose file, then choose encoding).

In Kate, we removed the choose encoding for open and rely on the detection and for save we have an extra action that let's you first select the encoding (which is rarely needed, just for some "convert file to X" tasks you do once a week ;=)

So, the task is rather port apps away from KEncodingFileDialog; using Ktexteditor/Kate as an example as to how to replace the functionality.

The apps that use it:
kdevelop
kile
skrooge
kgpg
kmail
messagelib
rkward
tikzkit (from playground)