Turns out Daffodil is not the only DFDL project creating extensions to DFDL.
In order that schemas that are portable/non-portable can be distinguished, the DFDL workgroup has decided to bless a specific extension namespace, which is the current dfdl namespace plus the word "extensions".
All the properties that we've added to dfdl that are beyond the DFDL v1.0 spec should appear in this new dfdlx namespace instead.
When they appear inside a dfdl:format or other long-form annotation, they would need the dfdlx namespace prefix, unlike standard dfdl properties which omit this prefix when used in long-format.
When referenced from <dfdl:property name="dfdlx:newProp">.... they would use a QName, not just the bare name as DFDL v1.0 properties do.
This same new prefix should be used for extension functions we add to DPath.
Since this change would break existing Daffodil schemas, it is sensible for both the current and new namespace both work for a while (co-exist) based on a tunable flag, and we issue a (suppressable) warning about the older style.