That's basically what a simple system like Hi-Lo already provides. To me, it seems more complicated and difficult to determine a percentage of high cards seen compared to total cards seen than to maintain a running count that does the same task.
The first idea requires you to keep two running totals: The number of high cards, and the number of total cards. Using a plus-minus system does the same thing with a single running count.
Would it work the first way? Sure, but with extra effort.