Is a speedbooster just a reverse mounted teleconverter?

Does my harebrained idea of adapting Pentax 67 lenses to a 35mm body stand any chance of success?

I don’t know what a speedbooster is, but Pentax sold an adapter to mount 67 lenses on Pentax K mount.

Bronson Dugnutt said:

Does my harebrained idea of adapting Pentax 67 lenses to a 35mm body stand any chance of success?

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Yes, just use the Pentax adapter made for that purpose but no speed will be boosted.

A speedbooster is, in essence, a kind of reverse teleconverter- its a converging group of lenses, rather than a diverging group, like a teleconverter. That doesn't mean that a speedbooster is literally a reversed teleconverter, or that you can achieve the same effect by mounting a teleconverter backwards, if that's what you're asking. It's certainly theoretically possible to design a lens group which adapts medium format lenses to 35mm in the same way as a speedbooster is used for APS-C and MFT, but like others have noted, I don't think such a thing exists currently, and adapters for P67 lenses and others generally don't have an optical component, they just get the flange to film distance right and make the mounts compatible.

Kipon Baveyes makes focal reducing/speedboosting adapters for medium format, including Pentax 67 to certain 35mm, mostly digital, mounts