Eh, the problem with that reasoning is one of extreme degree. The "obscurity metric" would be the surprisal associated with discovering the critical piece of info. Using a random port confers brute force resistance of 2^16. At 1ms that's about a minute. Brute forcing at the same rate a 128 bit key takes like 10^28 years.

It's like hiding your key under the mat, vs hanging on a tree limb of a specific tree only you know the gps coordinate of. Both are "obscure". Huge difference in difficulty.