I've been toying around with some baseball infields and have a few thoughts about this. First, here's what I've done to Doubleday Field in Cooperstown, NY:
I used multipolygon relations to recreate the field of play. The infield and outfield grass is tagged as "leisure=pitch" and "landcover=grass"; the grassy areas in foul ground are tagged only as "landcover=grass." I like the idea of drawing all of the grassy areas but wanted to differentiate between areas in play and areas out of play.
As for the basepaths and the infield dirt, I tagged them "baseball=infield" and "surface=dirt." Neither of these tags render in Mapnik (or MQ.open) but the infield dirt is clearly outlined between the infield and outfield grass. I even included the pitcher's mound, although I only tagged it "surface=dirt."
Any thoughts? I think this is the workflow I'm going to use going forward to map infields: create multipolygon relations for the grassy areas and the infield dirt/basepaths will be clearly outlined as a result, even without explicitly creating a multipolygon relation for it.
Edit: I actually decided to add the "leisure:pitch" tags to the grassy areas both in fair ground and in foul ground. Still separate multipolygons, so the Mapnik render still shows the "foul lines" in the outfield, but in the MQ.open render it all looked the same anyway. I like it better this way.