Here's a clever idea from
JED:
Cutting out sections of block divider, and inserting rebar for a reinforced lintel. Of course you have to support the span from below when you set it up, and fill it with concrete. A slightly more expensive idea is to buy a pre-cast concrete lintel to span the gap. You could do like I did, and make a form and build a masonry arch.
Or as christo says, you could just put an extra piece of rebar in the front of the slab, and forgo the lintel entirely.