According to https://access.redhat.com/solutions/4385291:
The None mode (SameSite=None) is still not available in EAP 7.2.x because SameSite=None is not yet defined in the spec or proposals. So, it's not possible to add SameSite=None even with the above custom handler.
It is not availible in EAP 7.3 (which is latest available version) also. Are there any plans to release patch for 7.3 with support of SameSite=None, introduced in Undertow 2.1?
UPD: Found Jiras to include it into 7.2.9 (https://issues.redhat.com/browse/JBEAP-19580) and 7.3.2 (https://issues.redhat.com/browse/JBEAP-19579)