D-STAR Digital Data as implemented by Icom uses the D-STAR
Frame for routing on air and from gateway to gateway. So you
will want to test before deploying a network in a mesh
environment.
I'm not sure if ID-1s treat UR:CQCQCQ as a broadcast for DD,
thus enabling an IP payload to transparently go to multiple
receivers. I don't know the answer, but I'd test well. The
test could be performed with two ID-1s on DD simplex (not RPS)
and setting both to UR:CQCQCQ and try to pass traffic. The
payload is Ethernet frames, so it should just act as an
Ethernet bridge over RF if the CQCQCQ D-STAR address functions
as broadcast.
The protocol itself is ambiguous on this point.