Which technology and VPN component allows a VPN headend to dynamically learn post NAT IP addresses of remote routers at different sites?

ISAKMP is a key management and negotiation protocol; it handles authentication and SA establishment but has no mechanism to register or resolve tunnel endpoint IP addresses.

GETVPN is designed for trusted MPLS cores where all devices share a single SA and routable addresses; it does not support dynamic NAT traversal or NHRP-style endpoint registration.

In a DMVPN topology, NHRP (Next Hop Resolution Protocol) functions as a dynamic address registration mechanism where each spoke registers its current public IP - including addresses translated by NAT - with the NHS running on the hub. This allows the hub to maintain an up-to-date mapping of spoke tunnel endpoints even when those IPs change, which is the core requirement for dynamic post-NAT IP learning.

GETVPN does not use NHRP and is architecturally incompatible with the dynamic post-NAT IP discovery described in the question.