In case anyone else using this feature is curious, I've compiled a list of missing features and bugs from our initial proof of concept phase and I've entered the tickets below.  I'm willing to help send pulls for some of these, though I am a little curious regarding the future of Undertow and if Undertow 3 is really happening any time soon at this point or if I should just be focusing on Undertow 2.x for the foreseeable future.

Also, on a side note, could a JIRA admin please add a "Predicate Language" component in JIRA for me to tag these tickets with?

Predicate Language Redirect handler doesn't allow custom status code
https://issues.redhat.com/browse/UNDERTOW-1734
Predicate Language handler to set inline response body
https://issues.redhat.com/browse/UNDERTOW-1736
Predicate Language response-code handler doesn't allow custom status text
https://issues.redhat.com/browse/UNDERTOW-1735
Predicate Language access-control handler doesn't have failure-status parameter
https://issues.redhat.com/browse/UNDERTOW-1737
Relax case sensitivity in Predicate Language for ease of use
https://issues.redhat.com/browse/UNDERTOW-1738
Sub groups don't work with nested predicate
https://issues.redhat.com/browse/UNDERTOW-1739
Predicate Language parsing fails on semi-colon before "else" keyword
https://issues.redhat.com/browse/UNDERTOW-1740