I don't believe there is any requirement for markings in that neutral area to begin with, so it's unlikely that you'll find any particular standards specifically for it. If Ontario wants to use that kind of striping, I don't see any rule (Canadian or American) saying "no"; my suggestion of using chevrons simply comes from neutral areas between same-direction traffic flows normally use chevron-style striping, if any striping is used at all.
Perhaps the closest analogy might actually be the striping in the neutral (buffer) space between regular traffic lanes and bike lanes. From speaking to an American-based professional in the field, the neutral space would have either no markings if 1 foot or less, regular diagonal striping if between 1 and 3 feet, and chevrons if greater than 3 feet in width. Applying this theory to the HOV lanes in Ontario, they seem to be about four feet wide, so they should use chevron markings. But since they are not bike lanes, and MTO probably has their own standard, it's a moot point anyways.