After the earlier brief discussion, I thought about the similarity between this feature and "properties" in Perl 6. I think it would be useful to design the front-end in such a way that it can be re-coded to use properties (mimicing the property syntax of Perl 6 directly is probably not practical without a source filter, and it may change before it sees first light anyway so it doesn't seem worth the effort, unless you want to specifically explore the Perl 6 feature in applied use before it's too late to redesign it).
I don't think the example at the end of your post is a good example. Why must your tuple be crammed into a simple scalar? That looks like the job for an iterator class based on a blessed array of two items, and possibly an overloaded stringize or numify operator.
—John