I was able to send metadata with H264 using GStreamer RTP lib's gst_rtp_buffer_add_extension_twobytes_header(). The rtph264pay is RFC 3984 has does support this.

But it does not work for PCM. Calling gst_rtp_buffer_add_extension_twobytes_header() in probes attached either rtpL16pay or rtpstreampay does not work. Likely because RFC 3551 or RFC4571 does not support header extensions.

Is there any other way to add arbitrary metadata to PCM over UDP/Network?