You might be able to set the PIO mode jumper and do it that way.
Apparently (and I'm only repeating what I have read in the manual and elsewhere) Buster 11 will only allow one DMA zorro device to operate at once. Since the internal scsi of the A4Kt operates as a DMA device, you will only be able to operate Deneb in PIO only mode whilst there are devices on the internal scsi bus.
Personally, I'd ask Mr Boehmer for advice, since I've not explicitly read anywhere that the "rescue" jumper and PIO jumper should be set at the same time. Logically it should be possible, since you must be able to upgrade the firmware in a non-DMA capable zorro environment (A2K, A1200, etc), but it wouldn't hurt to play it safe.
In the longer term, I'd look getting an accelerator with built-in scsi, and moving your devices onto that. That way you'll be able to operate the Deneb in DMA mode.
Regards
Rich