I would like to make a VERY small, lower power, USB over IP server. I know that Linux has a "usbip" package, but is this available on a microcontroller like the ESP32?
The client is running Linux, so usbip packages on client+server should make this easy. And, both client and server will be connected by Ethernet.
There is a proof of concept for ESP32. It is supposed to work with simple devices like HID.
No; as far as I'm aware the only compatible other end for the linux kernel's usbip module is for Windows, and the module inside the Linux kernel, and none of that runs on an ESP32. It's even in the FAQ:
Does your USB/IP implementation have compatibility with other similar products?
No, currently.
You'd need something that can run Linux, like a system-on-module by microchip, rockchip, or some kind of Linux-capable SoC board (like the raspberry pi and others).
The honest truth, though, is that USB-over-IP, and especially USB-over-IP-over-Wifi (considering you mention an ESP32) is in almost all cases the wrong choice. USB is a very "ping-pong" protocol on a physical layer. That means a lot of small packets, each of which must be requested by the host. That eats a lot of bandwidth and power, and it makes USB over high-latency networks unbearably slow, or even impossible, if the drivers talking to the USB device expect some benign maximum latency and otherwise assume malfunction.
In other words, no matter what you try, network is not a transparent transport for USB.
It's much more sensible to instead let the low-power remote device actually do the USB host role, and talk to the device, and then just provide the service that this device offers via a network-appropriate protocol. For example, if you want to plug in a USB storage device, let the low-power device run an SFTP (ssh), SMB/CIFS or NFS server. If you want to plug in a printer, let it run a print server; if you want to plug in a scanner, a scanner server; if you want to plug in a sound adapter, a sound server, and so on. That way, you're using the network with appropriately-sized packets (and every packet has high overhead), and the latency that limits throughput isn't the latency between two USB stacks plus network plus the actual USB latency, but only the latency between one USB stack and the device.
That needs much less resources. For some of these problems, there actually are microcontroller-targetting reference implementations that you can port to your specific ESP32.
usb huband also check yourUSB Tetheringwith your android mobile. Then testusbip