I’m not sure if this is the right forum but adding CoAP/CoAP support analogous to http/https but for both TCP/TLS & UDP/DTLS would be useful for testing.
-Jim

If you run CoAP over TCP (RFC 8323) with coaps+tcp:// (that is, CoAP over TCP secured with TLS), stunnel should work for you as it is.

Since CoAP does not support in-band security negotiation (security is selected via the URI scheme, similar to HTTP vs HTTPS), what you need is likely either:

Pros:
 - Fairly easy to implement.
 - Protocol agnostic (will encapsulate most UDP-based protocols, and not just CoAP).

Cons:
 - Does not support CoAP-specific end-to-end features enabled by OSCORE (for example, proxy caching of protected responses).
 - Requires OpenSSL, which is often too heavy for smaller IoT devices.

2. A dedicated CoAP↔OSCORE wrapper (analogous to stunnel as a TCP↔TLS wrapper).  This would not be a transparent tunnel, but a CoAP-aware wrapper that terminates and re-originates CoAP messages.

Pros:
 - Additional CoAP-specific features.
 - Can be implemented with a tiny cryptographic library.

Cons:
 - Way more complex to implement than just adding DTLS support to stunnel.
 - Only useful with CoAP.

Does my analysis make sense?  Please contact me directly if you consider sponsoring additional features.

Best regards,
    Mike