This was clarified on DFDL workgroup call on 2016-10-11

Verify that Daffodil conforms to erratum 5.18 per below.

lengthKind ' expression ' when unparsing

Spec says "When dfdl:lengthKind is 'explicit' (and dfdl:length is an expression), 'delimited',
'prefixed', 'pattern' the data value is padded to the length given by the XSD minLength facet
for type 'xs:string' or dfdl:textOutputMinLength property for other type" .

This is not the latest agreed thinking. The desired behaviour is as documented by erratum 5.18
in Experience Document 4, which states the expression is evaluated and the result taken as
the explicit length. This has not yet been added to the spec.