There is no physical 0.5 A "cap" on USB port power. Quite contrary, according to USB 2.0 specifications, Section 7.2.1, if a USB host is powered from AC or a big battery, so-called "high-power" host ports must supply AT LEAST "5 units of load". Since each "unit of load" is 100 mA for USB 2.0, a USB port must supply "at least" 500 mA, which means that it can supply more, usually much more, 2-2.5A, even if it is a normal, non-charging port. However, designers prefer to limit the individual port supply capability and supply only the required "at least 500 mA" plus some sizable overhead to accommodate manufacturing variability of cheap resettable fuses, to meet contact rating, and to avoid liabilities.
If you are making your own hub, you feel free to put any regulator, and supply whatever current you want, provided that your hub doesn't catch fire, because standard USB connectors have limited contact rating (which is set by overheating), and cable wires can be smaller than 28AWG in newer cables. Even if you find high-current USB receptacles, there is no guarantee that customer's cable plug will meet your desired rating, or will use full-size 5-m long cable.
If you are making a device that needs 800 mA, then you will be in trouble if a user of your device attempts to plug it into "barely powered" port, and you will have a lot of complaints, not mentioning that your device will be uncertifiable.
But if you making a paired hub and device, feel free to do whatever you need, just use proprietary connectors that support your power needs (and meet USB electrical quality requirements for differential impedance on data lines), and proper cables.