You can specify the radius around the path in tiles; the spacing would depend on that. Sorry, but I don't know what the tile size is on any zoom level.

My tendency, having access to CAD software, would be to draw the polygon outline in CAD, and offset it inward by the spacing, not to zigzag. Then connect all the tiers, join them all up, export the point list, and mess with awk to get a gpx file... Really, it'd be a worthwhile project to make a standalone triangle -> path converter, as it's trivial to divide a polygon into triangles (especially a convex polygon), and comparatively simple to make a triangle thing work.