Invalid syntax to cause runtime error in encoded-character

Kjetil,
We' ve discussed your suggested text for encoded-character Sieve 
extension during the San Diego meeting.
I think people are generally happy with your text (+ Philip's proposal 
for generic syntax for "magic things inside {}"), but both Ned and I 
don't think that invalid sequences should cause runtime errors. Can you 
elaborate on why you think causing runtime errors would be better than 
just leaving invalid sequences as is (like in variables)?
Thanks,
Alexey