I gave this a a try with WildFly 10.0.0.CR4 (Undertow 1.3.3) and it doesn't work yet. I've left a comment.
Furthermore it seems 4.2.2 no longer works with Undertow 1.3.3:
Caused by: java.lang.NoClassDefFoundError: org.xnio.StreamConnection from [Module "deployment.spring-websocket-portfolio.war:main" from Service Module Loader]
at com.sun.proxy.$Proxy41.<clinit>(Unknown Source)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
Moved to 4.3 RC1 due to Undertow's doUpgrade implementation not working with programmatic endpoints yet. This has been fixed in Undertow already but won't see the light of day until 1.3.5, so let's rather not make any bets and keep the fine-tuned version of our existing manual upgrade code (#18197) in place for the time being.
Rossen Stoyanchev. please roll your doUpgrade branch into master now... simply using that method when present, as originally intended. We're at Undertow 1.3.10 already, and WildFly 10 is still not GA, so it's not really relevant anymore whether 1.3.1-1.3.4 didn't properly work there... in particular not by the time we go 4.3 GA.