BJT's don't "clamp" on voltage. What you're reading is the maximum safe forward bias voltage they're rated at. They aren't designed to work like a Zener, so there's no real rating on how far they'll go when you push them beyond their rated specs. The PN junction of a BJT is far more similar to a PN diode (rectifier diode) than to a Zener diode, so really what you're doing here is looking at the breakdown voltage of the base-to-emitter junction. The life-expectancy of BJT's used in this manner will probably be pretty short on average. This is really outside their area. The Zener is designed to run high reverse bias voltages all day, every day, with (relatively*) predictable results.
* Zeners are good first-approximation, inexpensive voltage regulators. However, there's a reason you go to 78XX/79XX for more precise voltage regulation.
It looks like you got a bit of coverage on this on the SLO Clone forum when you brought it up. I usually don't read that subforum.