I'd rather skip my own arbiter if possible, but you gave me an idea.
What about 2 avalon slaves on the same avalon bus, each with a conduit to pass the control signals out to the same registers? The first master would only access the first slave device and the second master would only access the second slave device. That way I should be able to tell which device is accessing the registers and control the access, but still use the avalon arbiter.
The second device is the PCI core, so I would prefer to take advantage of all of the built in arbitration and control of the avalon interface.