The "telecompressor" you mentioned is a focal reducer, a device that concentrates the image in order to project it onto a smaller sensor.

This approach reduces the backfocus distance (the distance from the back of the lens to the sensor). So it only works on mirrorless cameras using lenses designed for DSLRs (which have sufficient backfocus distance to make room for the adaptor).

They do exist though, Metabones were first to market with this type of adapter for mirrorless cameras (though they've existed for telescopes for years). The focal reducer maintains the depth of field you get with the larger system, i.e. you can get the "full frame" DOF and bokeh using an APS-C sensor.

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It would be possible to design a focal reducer, or ground glass based approach like the one you describe for larger formats. No-one to my knowledge has bothered trying to do this, however. The reason for this is that contrary to popular opinion medium and large format systems don't really offer shallower depth of field compared to 35mm.

Technically a larger format gives shallower depth of field for the same f-stop, but that's exactly where the problem lies, you can't get lenses with the same f-stop for larger formats.

Medium format lenses are typically f/2.8 or slower, large format lenses tend to be for like f/5.6 or f/8. When you work out the depth of field equivalences they end up about equal, or more commonly worse than their 35mm format equivalents.

Sure there are a few exceptions, such as the large format 150mm f/2.8 Schneider Xenotar (which you can't afford), or the 178mm f/2.5 Kodak Aero Ektar (which is radioactive) but on the whole there would be very little to be gained by the time you've taken the losses of the adapter / ground glass into account. Lenses on the speedbooster show quite minimal loss of sharpness, however that's only a 0.71x, for large format you'd need a 0.28x!)