Yes - one of my purchases worked. This gives me USB-to-serial, without crashing the Raspberry Pi. As a bonus, I get *two* serial lines.
So I've got it running off the Geocaching Robot Arm, and it controls the relay box that switches light and power on and off. The same relay box also controls power and light on the forthcoming Geocaching Robot Arm Two (which was a bit delayed when the circuit board controlling it stopped working, but I'll be able to get round that by using another relay box).
Since there's a second serial port going begging, I'm using that to communicate with the relay box that will be operating the Geocaching Robot Arm Two.
Which leaves the Dr Solomon's Railway still being switched by a Tower PC. It's too far to use the Robot Arm switching Box to switch that; ladysolly wouldn't appreciate me running lines all over the house for everyone to trip over. So I've ordered a couple more of the USB-to-serial thingies that I know know works; one for the Railway, and another one just on general principles that I'm sure to find a use for it. I have a few more relay boxes unused, left over from a previous project.
So next I'm going to have a look at the USB relay board I just got. It comes without doumentation, and the software you get when you download the Zip file is for Windows, but I'm somewhat hopeful.