I wrote an alternative firmware for my HybridSID device: It can now act as a 'Network SID Device' and can be used as the sound output device for 'ACID 64'  and 'GoatTracker' .
I also implemented a small "dual boot" firmware which runs at power-on and which allows selecting the desired use case by pressing the left or right button: Pressing the left button starts in the stand-alone 'HybridSID' mode, pressing the right button starts the 'Network SID Device' mode where the device receives the register values for the SID chip from the application running on a PC.
Beside the disadvantage of not being stand-alone, the 'Network SID Device' mode has several benefits:
* Newer tunes containing samples are only released in RSID format which the TinySID library used in stand-alone mode cannot play. ACID 64 plays them nicely.
* ACID 64 sends the cycle information together with the register values, so it is possible to write the registers of the SID at the precise time. The new firmware is cycle exact.
* Easier tune selection due to the larger PC screen and richer GUI program. Also the STIL information can be viewed.
* ACID 64 has a nice song position slider to jump to an arbitrary time-position in the tune.
ACID 64 sends the data to a socket connection on port 6581. Traditionally, there is 'Java SIDplay2'  listening on this port, so ACID 64 can be used to drive this sophisticated SID emulator.
To get ACID 64 talking to my device, I wrote a small socket server in Python which handles the socket communication with ACID 64, and writes the received cycle info and register values to my device. This is done through an emulated serial port which the 'Network SID Device' firmware provides via a CDC/ACM USB device class.
By default the device buffers 100 ms worth of data which can be changed by turning the rotary encoder.
YouTube video: http://www.youtube.com/watch?v=mimSNZdvRj4
Please find the source code (firmware.zip) attached to this post.
UPDATE: firmware_2011-10-02.zip fixes some bugs and adds PAL/NTSC clock switching.