I created a test that uses inputValueCalc to set the value to '{' by using '{{', which, according to DFDL-6-078R should work and not be treated as an expression.

However, I get the following error:

Error: Value '{{' is not facet-valid with respect to pattern '({[^\{].*})|({})' for type 'DFDLExpression'

The test is expressionRules03 in
daffodil-test/src/test/resources/daffodil/section23/dfdl_expressions/expressions.tdml

I currently have the element commented out because it seems to break the whole schema.

I looked to see if this is just something that has not yet been implemented, but didn't see an existing ticket. Feel free to close as duplicate if I just missed it.