I also do not understand why USBHost and Wifi TCP/UDP server were never implemented but it looks like Mbed decided to be device/client in many cases probably in a context of IOT.
In this topic some one else also tried something like you but with ST target. So if you will be lucky it will work also for LPC1768. And how I see, the library still contains HALs for LPC1768. So maybe you can try to create a new project based on MbedOS 5.15 and use USBHOST - USBHOST lib for STM | Mbed
If this will not work, then you can try a luck with the old library and old Mbed2 via Keil Studio Cloud where is Mbed2 still supported in limited mode.
BR, Jan