First you have to decide or specify what you want to size & lube those ~ 200 grain cast bullets for…
SAAMI specs call for a .350 bore with a .3595 groove diameter… on the 38 S&W…
Once you decide on the final size diameter one can then look at what is available in a pistol expander plug that might work to expand those pistol cartridge cases to what you are looking for…
There is a different thread on the NOE forum that tends to cover expanders in general & pretty much explains the size ranges of both the rifle & pistol expanders that might be of use here to you and to other members of the forum…
Once a base line has been established one can then attempt to gather support for that endeavor to include in the pistol case expanders the proper sized expander plug.
Below is the link to page 1 of those pistol plug expanders…
Potentially decide on a sizing diameter then one can possibly gather some support for your request… for a pistol expander plug for the 38 S&W…