Well, after waiting after the big "R", there seems that ATN and DATA
were both low, at least for a small amount of time.
I've run the cbmlinetester tests again, with the ZF connected to the
1570 and the 1570 connected to the 1541. The results are in the Google
Sheet I've linked to previously. The results now seem much more sane,
with ATN/DATA turning on and off as would be expected.

matthias@MacBook-Pro-von-Matthias opencbm % cbmctrl status 8
99, driver error,00,00
matthias@MacBook-Pro-von-Matthias opencbm % cbmlinetester -i
[XUM1541] found xu/xum1541 version 0208 on bus 250, device 5
[XUM1541] xum1541 name: xum1541 floppy adapter (ZOOMFLOPPY)
[XUM1541] xum1541 serial number: 0
[XUM1541] firmware version 8, library version 8
[XUM1541] device capabilities 1b status 00
[XUM1541] [xum1541_init] Tape supported, disk mode entered.
[XUM1541] firmware git revision is 3ef4fc0d
[XUM1541] compiled with avr-gcc version 5.4.0
[XUM1541] and using avr-libc version 2.0.0
[XUM1541] write 16 2 bytes from address 0x7ff7bd03e5ee flags 3
[XUM1541] wrote 2 bytes (48 6f)
[XUM1541] xum1541_wait_status checking for status
[XUM1541] return val = 0
[XUM1541] wait done, extended status 0
[XUM1541] write done, got 0 bytes
99, driver error,00,00
[XUM1541] Closing USB link
% matthias@MacBook-Pro-von-Matthias opencbm %
matthias@MacBook-Pro-von-Matthias opencbm % cbmlinetester -i
99, driver error,00,00
[XUM1541] Closing USB link
% matthias@MacBook-Pro-von-Matthias opencbm % XUM1541_DEBUG=9 cbmctrl status 8
[XUM1541] scanning usb ...
[XUM1541] device 16d0:0504
[XUM1541] found xu/xum1541 version 0208 on bus 250, device 5
[XUM1541] xum1541 name: xum1541 floppy adapter (ZOOMFLOPPY)
[XUM1541] xum1541 serial number: 0
[XUM1541] firmware version 8, library version 8
[XUM1541] device capabilities 1b status 00
[XUM1541] [xum1541_init] Tape supported, disk mode entered.
[XUM1541] firmware git revision is 3ef4fc0d
[XUM1541] compiled with avr-gcc version 5.4.0
[XUM1541] and using avr-libc version 2.0.0
[XUM1541] write 16 2 bytes from address 0x7ff7ba47f5ee flags 3
[XUM1541] wrote 2 bytes (48 6f)
[XUM1541] xum1541_wait_status checking for status
matthias@MacBook-Pro-von-Matthias opencbm % cbmlinetester -i
own: 0000:
line status: 0017: ATN DATA CLOCK SRQ
Drive powered on:
matthias@MacBook-Pro-von-Matthias opencbm % cbmlinetester -i
own: 0000:
line status: 0012: CLOCK SRQ
Typing a and A or d and D causes each line high or low corresponding.
Any clue to this?
M.