I wonder, if it would be worthwhile to find a local machine shop and have them drill the new end caps and press fit standard freeze plugs into them? Just in case there's a "next time". Should be a fairly simple job, assuming you can find a machine shop these days.
The ends are relatively soft and it calls for only 2ftp of torque. I don't think a freeze plug would work. The main engine has freeze plugs, but I think these ends are a type of freeze plug the way I saw them react when getting frozen.
Nice idea though.