[LAD] Latency measurements: External round-trip vs. absolute in/out values

[Tim]

With a discussion about latency in the Jack lists, I thought
  I ask a question I've been wondering:

By connecting a cable from a HW output to a HW input, one
  can measure the round-trip latency.

But if the purpose is to try to determine the absolute
  output latencies of the output port and the input port,
  by say, dividing the round-trip value by 2 and assuming
  this value applies to both ports, then won't this concept
  fail if the output port has a different latency than the
  input port? Say, by using two different devices for the
  input and output ports.

I find myself asking what exactly transport position is and
  how it relates to the data in the outside physical world.
I wonder if it would make sense to build an external
  hardware transport position indicator so that the latency
  test signals could be compared with it.
For example to measure an output port's latency:

     ------>--[HW transport position indicator]-->---
[PC]-------                                        |
     ---   |                                        |
       |   -->-[Audio output port]-------->---[Comparator]
       |                                            |
       -----<--[Delay measurement]-------<-----------

The remaining unknown - the input port latency would be
given by subtracting this measured value from the total
  round-trip latency.

Sound crazy?
Thanks.
Tim.