To give some perspective, the Bicicapace Classic has a reinforced front bag that is large enough to carry a folded Brompton. It's actually really quite large.
The Brompton front block is rated for a comfortable 10kg I think. I once carried more like 20kg on it, with an extra webbing strap routed through the frame triangle, and not at any high speed. The strength of the block is limited by about 6mm of thread of two M5 bolts. The luggage block mount itself, which is brazed onto the headtube, is actually capable of supporting an adult* cantilevered out three feet.
* I believe the test subject was Andrew Ritchie.