This is true for decagon devices, I'd forgotten about it as I was also working with a stevens hydra probe (SDI version) which doesn't have a friendly DDI startup. Actually the decagon manuals have a very nice description of SDI12, easier to read I think than the official specification (I do see that the manual for the 5TE and GS3 disagree as to whether DDI is active high[yes] or active low[no]).. Of course they insist on a nonstandard 3.3V, but that can be advantageous nowadays.
As long as you only need the measurements that are pre-programmed for the DDI burst, you're golden. A multiplexer would work great, but you'll also need to be able to supply and remove power to toggle the measurements.