First of all, let me just say that it sounds like you have an outstanding system for seahorses! Tremendous water volume, loads of live rock, and a large refugium should all help keep your seahorses fat and happy.
I think your theory is perfectly sound and that it’s quite likely that the seahorses have been subsisting on the natural fauna in your tank up to now. Although captive bred seahorses are trained to eat enriched frozen Mysis as their staple diet, if given a choice between frozen food and live prey — especially juicy copepods and amphipods — they are likely to choose the latter. Nothing stimulates a seahorse’s feeding instincts like the frantic movements and evasive maneuvers of real, live, "catch-me-if-you-can" prey items!
If the natural fodder in your system can sustain mandarin dragonets without any supplemental feedings, I think it’s also safe to say that it can also support a pair of seahorses which prefer to graze on the ‘pods in their partition, at least for the time being.
There is indeed an easy way to determine whether the seahorses are getting enough to eat on their own. Just make sure all your seahorses have full bellies at the end of the day, as indicated by their well-rounded abdomens. After a good feeding, the seahorses belly rings should be flush or even slightly convex in cross section when viewed from head on. (We never want to see sunken, severely pinched-in abdomens on our seahorses! Concave belly rings are a sure sign of an underfed seahorse, with the sole exception of a female that has just transferred her eggs.)
So if you want to check whether your seahorses are eating well are not, Don, don’t look at their profile — examine them head-on. Their abdomens or belly plates should bulge out slightly or at least be flush with their flanks, not pinched in or sunken. In other words, when viewed from the back or from head-on, the cross-section of their abdomens should appear concave "( )" or flush "l l" rather than concave ") (" or pinched in.
In my opinion, the best way to interest them in frozen Mysis is to train them to use a feeding station. I wrote an article in Conscientious Aquarist explaining how to do exactly that in some detail that may be worth checking out. It’s available online at the following URL:
Click here: CACover
Also, have you seen the club’s usual feeding tips yet, Don? If not, I would be happy to repeat them for you here. They include suggestions on enriching, feeding new arrivals, secretive feeders, target feeding, feeding stations and more, which could be helpful in your case.
Best of luck with your new Sunbursts, Don!