im using deneb setup mostly with aos, poseidon didnt work with aros yet.
AROS has Poseidon built in though. Is the USB support buggy or are drivers missing for your Deneb? Have you tried the AmigaOS Deneb driver under AROS? I believe the communication for input (mouse, keyboard, gameport) would be Deneb driver to input.device. The Deneb driver should work with the minimal AmigaOS 3.x input.device of which the AROS input.device should be compatible. Poseidon for AmigaOS does include the MOS version of input.device (68k compiled and optional) which has new MOS style device commands which give some new functionality, especially for pre-boot like using the mouse or keyboard in the early startup menu. The AROS input.device decided to use the AmigaOS 4.x style device commands (actually only 1 for now) and modified their input.device and the AROS version of Poseidon for this. It is possible your Deneb device would partially work with AROS (as well as the AmigaOS 3.x input.device). It might be worth a try.
I recently sent Gulliver a new input.device I have been working on to test. It is currently compatible with the MOS style commands for AmigaOS Poseidon compatibility but could be changed to the AROS/AmigaOS 4.x style. This may allow for porting the AROS version of Poseidon back to AmigaOS 3.x and/or allow the new input.device to run under AROS (for test purposes and perhaps better optimization). As is, it should be a drop in replacement for the input.device included with Poseidon. It requires Remus to install in a custom kickstart. It saves some valuable kickstart space as this new input.device is 3768 bytes vs 5544 bytes for the Poseidon input.device. Let me know if you want to test it.