The accelerant interface is not really designed to add arbitrary hooks this way.
For the intel driver, we have a specific kernel debugger command (ie_reg) that allows reading and writing video card registers. For the accelerant it depends what you are doing: if you are using it in app_server, maybe the easiest way to reach it is to have some kind of IPC in place in your accelerant, either run a BLooper to receive BMessages, or open a socket. Then your test application can send messages there.
If you do your tests without app_server, I assume you will load your accelerant with load_add_on and at that point you can access any function and variable in it directly.