I managed to get the dynamic header configuration at least with inbound proxies:
DATA:
server_context TYPE REF TO if_ws_server_context,
proxy_framework TYPE REF TO if_proxy_framework,
internal_protocol TYPE REF TO if_wsprotocol_internal,
lo_inb_pxy TYPE REF TO cl_xms_inbound_proxy_plsrv,
lo_dyn_header TYPE REF TO if_xms_msghdr30_dynamic.
server_context = cl_proxy_access=>get_server_context( ).
internal_protocol ?= server_context->get_protocol( 'INTERNAL' ).
lo_inb_pxy ?= internal_protocol->get_framework( ).
lo_dyn_header ?= lo_inb_pxy->xmb_message->get_dynamic_header( ).