I get most of my left hand muting from my middle finger fully stretched out, laying right up agains the board and the strings. This does, however, pose some minor complications. For instance, lines that you would default to fretting predominately with your middle finger while playing fingerstyle need to be approaches somewhat different with the left hand.
It is, for me at least, worth it to clean up my playing even though there are a few lines that my fretting slows down a bit on. Even so, I think it's worth it because it really isn't impressive to play fast and have it sound like a mess, its impressive when you can pull it off smoothly and cleanly. I use the middle finger to do this because, not only is it the longest and lays the cleanest across all the strings, but also because it still leaves the width of my hand to fret with (say, as opposed to choosing index, which would then limit my reach all together, rather than just in some combinations).
Give it a try, may work, may not - it appears that slap, perhaps more so than any style, is highly dependent on what works for the individual person.