You can't design a retargetable UI TO A BLANK PIECE OF PAPER.
Take as an example from another problem space, the Linux Kernel...
The Linux kernel was designed for one CPU. The Intel 80386. This vastly simplified the program's overall design. However, Linus made some basic assumptions about memory architecture, and tried to make an API to suit them, but these things happened _AFTER_ the initial design. Had he made the decisions BEFORE the completion of the first iteration of the design, things would have turned out very differently for Linux and its design... The results of his initial quick solidification of assumptions have proven to be scalable over time, as Linux runs on every CPU architecture with SOME form of memory management (and even some without..)
Remember, DCE is a messaging infrastructure. You all are placing too much creedence on Orbiter's importance. Even as it is right now, as long as the replacement is a DCE device, data grids can be pulled for dynamic information, scenarios can be queried for scenario data, and as long as there is a functioning DCE device at the Orbiter end, then the Orbiter Plugin can figure out where to send things.
So trying to think of everything to be overly abstracted and perfect...be careful...remember, right now, everything IS highly abstracted, and Pluto was able to make orbiters for TiVO and Moxi and Hillcrest, which use drastically different UIs.