I'm finding that many questions seem to be answered by LIFT the UNIVERSE. The answers additionally seem to be mostly good.
Here it wouldn't be possible, unless you use ^META assignments, in which it is the explicit job of those sinks to decide if they're willing to store the unstable antiform or not. (e.g. BLOCK!s don't, OBJECT!s do, etc.)
Your hypothetical pack-summer could exist, it would just have to say ps.^x: pack [10 20] to receive the unstable form.
SHOULD it exist? 
I don't think so.
The low-level assignment operator presumes you stored the bits as given. But this could change for any reason.. e.g. if you assign env.PATH: %some-path/ the synthesized value of that expression witll always match the right hand side. But other processes could run and env.PATH could give another value entirely on the next fetch.
Your pack-summer would similarly not be able to make the result of ps.^x: pack [10 20] return anything other than the pack [10 20], because that's the contract of the assignment operator and beyond the control of the target. But the next time you ask for ps.x I guess it can say whatever it wants... including but not limited to the sum of the members of the last assignment's pack.
Until I can think of a good counterexample, I don't think that assignments should be explicitly looking for ways to "get creative" at doing things that don't model value-preserving assignment.
Yet if you use a ^META assignment, then essential to the design is that it can see unstable isotopes now. I can't think of a good way to stop anyone from implementing features that don't model assignments... besides asking nicely not to.
But I don't need psychic powers to tell you that ACCESSOR, GETTER, SETTER are going to be used to implement people's arbitrary wacky ideas. Maybe I should ask the AIs if there are good examples.