It seems doing 1. does not prevent us from doing 2. later if we want to introduce macros. Am I right?
If true, then I lean towards 1. first to get something out. Then we can consider either 3 or 2.

My feeling about literal handling vs not is that we could start with a sufficiently unique escape sequence to make it unlikely and thus start with 3 before handling 2 which has to bring some primitive form of parsing at the very least.

Btw Sanne, I disagree with you, the syntax proposed by Gunnar is valid escaping {code}}

 is your exit sequence. Personally I think that something not involving bracket would be better due to the pervasive nature of JSON these days. 

|param:phone|

or something approaching would feel safer.

By the way, I suspect we always know the input type and we could use the grid types to convert objects like Date into their native string representation. Any complication with that? I've problem as I mentioned in the discussion is that some parameters require to be quoted as some might not depending on their types (strings vs numbers).