Depends on how you define "work". One key difference is that a true riving
knife moves up and down as the blade is raised and lowered, whereas a splitter
does not -- and hence your intended faux-riving knife won't either. In other
words, you'll still have to remove it to cut grooves, dados, or rabbets.

And you'll have no blade guard, unless you buy (or build) an over-arm guard of
some sort.

Seems to me that your plan combines the disadvantages of both setups, with few
(if any) of the advantages of either.

Yeah, I suppose that's true - I was looking at this as a slightly
better option than nothing at all. Most blade guard/splitters on new
saws are such crap that they are not worth using (yes, that's
debatable, and I wasn't looking to start that discussion...) Maybe I'd
be better to phrase this as taking a splitter that would otherwise
never get used, and reducing it to something smaller/more elegant that
would still serve to prevent a workpiece from pinching behind the
blade. A better, although not free, implementation of this would be
one of the table inserts that has a small splitter.

Lately, I've been seeing a bunch of saws that have a 'riving knife',
but one that does not move up/down along with the blade. From what
I've read, new model saws are going to be required to have a riving
knife in order to get UL listing. I wonder if the fixed position
knives are an inexpensive way to comply with the new UL regulation.