This is a bit of a kludge, but it should work okay if you're only using it for small files.
Drawbacks:
[ul][li]No confirmation before it replaces any files. It'll just do it. (Maybe not a drawback, depending on what you want. 
)[/li]
[li]It will copy over all the files in parallel. Not a problem with a small replacement file, but would be bad if the replacement file was large.[/li][/ul]
@set NewFile = {dlgopen|Select replacement file}
dopusrt /cmd Copy FILE {$NewFile} TO {filepath$|..} AS {file$} WHENEXISTS=replace QUEUE=none
(If you do need it to work with larger files, that can be done by queueing up each operation. The problem with that is there is a short delay between each file, which isn't a problem with large files but would be really annoying for the small-file case, since they delay would take longer than actually copying the file.)
To use this version, you'll need the replacement file to exist somewhere (not just data in the clipboard).
Select the files you want to replace, then run the command. It will prompt you for the replacement file, and will then copy over all of the selected files.
(Here's a short guide to creating buttons for commands, in case you need it. It's the "raw commands" part at the top that you need in this case; you can ignore the rest.)