What is the closure size impact? If it's small we might as well enable this by default.

Not sure if I'm doing something wrong, but it reports 154.82 MB for both closures. I ran nix-build -I nixpkgs=/path/to/nixpkgs -I nixpkgs-overlays=/ '<nixpkgs>' -A freerdp && nix path-info -S ./result. Is there something wrong with that?

Looks like the build system cannot find udev and dbus-glib:

-- Adding dynamic channel client "urbdrc": USB Devices Virtual Channel Extension
-- UDev not found.
-- UDev: You can specify includes: -DUDEV_PATH_INCLUDES=/var/empty/udev/include
--  currently found includes: UDEV_INCLUDE_DIR-NOTFOUND
-- UDev: You can specify libs: -DUDEV_PATH_LIB=/var/empty/udev/lib
--  currently found libs: UDEV_LIBRARIES-NOTFOUND
-- Could NOT find dbus-glib (missing: DBUS_GLIB_INCLUDE_DIR) 
-- Adding dynamic channel client "video": Video optimized remoting Virtual Channel Extension
-- Could NOT find DocBook XSL stylesheets (missing: DOCBOOKXSL_DIR) 

I am not very familiar with CMake but I am almost certain dependencies are not supposed to be found using find_path:

https://github.com/FreeRDP/FreeRDP/blob/eb1727bb875f27d7e02d47e0cc02f2e6546153e1/cmake/FindUDev.cmake
https://github.com/FreeRDP/FreeRDP/blob/eb1727bb875f27d7e02d47e0cc02f2e6546153e1/cmake/FindDbusGlib.cmake

You don't need this any longer?

No, I use ~2 RDP sessions per year and the USB-forwarding stuff was just a one-time thing

What hardware is needed to test this out? It would make sense to get this functionality in.

Well, any USB device that you don't need on your host. In my case, it was a signature pad, but as it turns out, it cannot be RDP forwarded, even with the native Microsoft mstsc client.
A USB thumbdrive would probably be good enough for testing