Use laser toner for embossing powder?
You should not handle toner powder at all without a high quality filter mask and potentially safety glasses. Loose toner is a health risk for the respiratory system due to it's particle size. It's not toxic, but some studies compare it's effects to the fine dust of diesel exhaust or even asbestos. You should at least expect an irritation or even inflammation of the upper respiratory system. (Source1, source2)

In any case, have a look at the material safety data sheet of the toner cartridge you want to open before exposing yourself and others to the powder.

Apart from the health issues, the melting tempetature of toner is 120°C / 250°F (which is lower than that of embossing powder) and at that temperature it should have a viscosity of oil (Source, PDF). But I doubt that you could cast anything in toner, especially left-over toner from an empty cartridge, because the volume of melted toner is much less than the powder would suggest.

I have no experience with embossing, but as far as I know it works pretty much like laser printing. You make a powder stick to certain areas on paper, then melt the powder to fuse it with the paper. You should be able to achieve the same effect with toner. I just propose using a heat source that doesn't blow the toner all over the place like a heat gun would do. Due to the small particle size I wouldn't expect it to feel any different from a laser printed paper.

A more practical approach could be binding the toner in a liquid and using it like an ink. You should be able to stack up a thicker layer of toner that way and achieve the desired 3 dimentional effect. In this video she mixes loose toner powder with isopropyl alcohol and a bit of school glue to get a fairly liquid ink / paint. And in this one she uses a different recipe for toner paint, and at around 13:20 minutes you see how shiny the toner got from being melted.