JPW,
Actually Barry in his DCS O Gauge Companion Vol 2 book, shows a trick where you can use many different Z4K transformers, I will not post the technique because MTH does not approve it, however it can be done and done safely, if you know what you must do to set it up. I used 6 different transformers at different times, on one DCS TIU, I did finally break down and purchase another full DCS with TIU, and now can run 8 transformers all at one time, from each of the 4 channels, on the 2 different TIU's, in an approved manner.
JPW if you do not own Barry's DCS O Gauge Companion and the OGR Guide to DCS,
I recommend you get them both.
PCRR/Dave