In theory ovs-vswitchd should already be reporting what kind of hash the datapath supports through the "Datapath.capabilities:max_hash_alg" key:
https://github.com/openvswitch/ovs/blob/d70688a7291edb432fd66b9230a92842fcfd3607/vswitchd/vswitch.xml#L6310

In practice this will always return 0 (OVS_HASH_ALG_L4) for the kernel datapath:
https://github.com/openvswitch/ovs/blob/d70688a7291edb432fd66b9230a92842fcfd3607/ofproto/ofproto-dpif.c#L1439

/* Data path hash algorithm for computing Datapath hash.
 *
 * The algorithm type only specifies the fields in a flow
 * will be used as part of the hash. Each datapath is free
 * to use its own hash algorithm. The hash value will be
 * opaque to the user space daemon.
 */
enum ovs_hash_alg {
	OVS_HASH_ALG_L4,
#ifndef __KERNEL__
	OVS_HASH_ALG_SYM_L4,
#endif
	__OVS_HASH_MAX
};

I'm not sure if the stable hash we need for the kernel datapath will end up being a symmetric one but if it does we could try to just use the Datapath.capabilities:max_hash_alg for notifying the controller.

As Comment 1 notes, this feature already exists for advertising the correct hash.