I could add a vertical peg or a lag bolt linking the beam to the support block. Limited opportunities for placement: center of hole would be about 1" from end of beam. Calculation with a 1/4" lag bolt gives what seems like an absurdly small load capacity:
AWC Connection Calculator
ASD lateral loading, lag screw, single shear; mixed oak of 2.5" thickness for both members; main loaded at 90, side at 0 degrees to grain; 1/4 x 4" lag bolt gives 140 lbs. Value doesn't change when both angles are 90 degrees, which seems very wrong...

Thoughts? (Wrong use of the calculator?)

Calculation based on shear out of a bolt gives about: 2 (shear planes) x 1" (from end) x 2.5" (deep) x 200 psi (Fv) = 1,000 lbs. That would get me where I need to be.

How would a peg do? I haven't paid much attention to how big a peg should be, but 3/4 seems like a minimum size, and there seems (intuitively) not enough relish---in the 2.5" total width I have to work with---to take a peg.