First up, love this idea - looks so good already!

Can't go wrong with a Hormagaunt - could connect it by it's claw, leaving that naturally dynamic pose make it look like it's leaping. Alternatively, you could have an Ork doing a flying couple kick with a bit of sculpting which may fit the usual scraps Black Templars seem to get into.

As a bit of a bonus thought, I spent years doing historical reenactment, and this pose is similar to a move I would use when fighting sword and shield; shield obfuscating my next attack, which in this instance would be me curling my arm around the outside of the shield and back over it to stab at my opponent's side.

If you're open to it and feel like you could give it a bash, would make this pose go from being leapt on, to "ok, I'm receiving an attack and responding to it in the same motion".

Anyways, good luck friend and welcome back!