Hans de Goede informs me that he has been working on gm12u320-kms: a new linux kernel driver for the Acer C120 mini projector, and possibly other devices based on the Grain Media GM12U320 USB3 bridge chip. Hans' driver exposes the projector as a KMS device and has been based on Viacheslav Nurmekhamitov's famebuffer driver.
The most interesting feature of this new driver is the support for PRIME: a cross-device buffer sharing mechanism provided by the linux DRM subsystem.
These USB displays/projectors are usually rather dumb, they just display frames passed to them in a given format, without any hardware acceleration; so PRIME is really useful for them, it allows the rendering to be performed by another (hardware accelerated) device, using the projector just for the final copy to the "screen".
Before anyone asks, this driver does not work with am7xxx devices.