Are there pre-synaptic alpha 2 receptors in the post-ganglionic synapses of the heart?
The heart of course does have Alpha 2 pre synaptic receptors , the definition states that alpha 2 receptors are present on sympathetic postganglionic endings , so I don't see why heart should be an exception . 
I will explain to you how reflex tachycardia occurs .
When you give non selective blockers , presynaptic Alpha 2 are blocked , therefore the pre synaptic neurons cannot sense NE amount in synaptic cleft , so they release large amounts of NE . BUT , post synaptic alpha1 is also blocked , so this large amount of NE cannot act BUT THE HEART DOES NOT HAVE alpha 1 it has BETA 1 receptors which are perfectly capable of being stimulated by this large amount of NE that is released , hence causing tachycardia.