Hi,
It sounds like it would work. It looks like a similar method is used on the Practical Paracord slings based on pictures.
So far what I've done for finger loops on 4-strand slings is twisted 2 strands together and folded them over to make a loop. The problem with this, however, is the loop twists and constricts your finger. Plus, it is quite thin, so it really digs in. This method has nothing to separate the loop and braid like your matthew walker knot would, furthering the finger-constriction.
I think that the method you describe would be effective and create a comfortable and good looking finger loop
(I might have to use it for future slings, as it sounds better than my current method
Thanks for the post!)