In case anyone is interested, it rained again so I made another trip to the same stretch of mud that locked up the front wheel before to see how it performed.
No problems this time, the space between the tire and fender stayed clear...
...and the upper fender worked well to catch the spraying mud from the shortened lower. You can see how the mud collected up at the front of the top fender; even though it is short, this was enough to keep any from coming back on me while riding.
So I'd say this setup works pretty well, and certainly is easy and cheap.