I agree. We all are aware of the psychologically-reinforcing component of injection drug use. Knowing this, a lot of things are possible. Folks will inject water for the Pavlovian response it can provide. It's easy to see how people would gravitate toward this, but again, people inject water so that's not saying too much.
It was once much more common for Opiate users to inject antihistamines with their drugs. In the United Kingdom (and I assume other places) Methadone was often dispensed alongside Cyclizine, as patients desired the added sensation when the two were injected. I enjoy 50mg Diphenhydramine (Benadryl) with an Opiate myself. It cuts down on the itching and provides a very mild additive effect to the Opiate.
I can see why people do it, though simultaneous use of Opiates and antihistamines is not a revolutionary change from Opiates alone.
Ive heard cyclizine is supposed to be the bomb with opiates i wish you could get it in Canada.