Set modtime of a local file?

[Kai Grossjohann]

I'm trying to implement the KEEP-DATE flag for the copy-file operation
in Tramp.  Depending on circumstances, Tramp reads the source file
into a buffer, then uses write-region to write the target file.

If the source file is local and the target file is remote, then it
works to find out the last modtime of the source file and then invoke
"touch" on the target file.  Tramp assumes that the remote system is
Unix-like.

But if the source file is remote and the target file is local, then
I'm stuck.  After using write-region to write the buffer contents,
what can I do to make the file on disk match the (remote) source file?

tia,
Kai