Regarding decals - I don't know whether that Superscale sheet can still be found or not. I bought mine a long time ago. As I understand it, Superscale nowadays doesn't do repeat runs of aircraft decals; when a particular sheet is sold out, it's gone. A few minutes ago I took a stab at finding that sheet on the Squadron Mail Order site; no luck. I suspect quite a few copies are knocking around hobby shops and the private stashes of aircraft modelers. I'm no expert at locating obscure aircraft decal sheets; maybe somebody in the aircraft section of the Forum could help.
The sheet was indeed labeled "1/72 scale." But it covered quite a few different airplanes and helicopters, and included shields and lettering in quite a few sizes. Whether one of them would work for a 1/350 Eagle I don't know.
When I was doing drawings for the Coast Guard Historian's Office I had occasion to get shown around the Coast Guard Air Station at Elizabeth City, NC. I think that was in either 1990 or early 1991 (I'm not a hundred percent sure). The station had just taken delivery on a brand new, shiny Hercules, which had "U.S. Coast Guard" painted on the fuselage. The other aircraft on the station (mainly Pelican helicopters) just had "Coast Guard" painted on them. The explanation, according to the pilot who was showing me around, was that George Bush's Secretary of Transportation had directed that "U.S." be added to the logo. Newly-acquired aircraft were painted that way, and older ones were getting the "U.S." added as they got repainted in the course of maintenance. I imagine ships were being treated the same way. The Superscale sheet was issued a few years earlier (I don't know exactly when), so it doesn't have the "U.S." As I remember, though, there are several copies of the lettering in each size on the sheet. So an Eagle modeler would have a couple of options: depict her in her pre-first-Bush-administration configuration (she would have had the "slash," shield, and "Coast Guard" from 1976 until at least 1989), or scavenge a U out of GUARD and an S out of COAST.
Another solution, if your computer has the Zurich Bold Extended font, would be to buy some decal paper and make the lettering yourself. Nitpickers beware: that's a pretty distinctive font. Several more common ones look almost like it, but not quite. To find out whether a model of a Coast Guard aircraft or vessel really has the right lettering, take a close look at the lower half of the capital G.
I'm a big believer in cross-pollenation in the hobby; I think aircraft, ship, armor, figure, and railroad modelers have a lot to learn from each other. But cross-pollenation does have its limits. Some years ago I worked as a judge in a ship model competition that emphasized Coast Guard-related models. One guy had entered a nice model of some modern CG vessel or other (I have no idea which one) with a beautifully-rendered "slash" and shield on the bow. The modeler's only problem was that he hadn't figured on getting a near-sighted judge. (I have trouble recognizing my wife from across a room, but until recently I could do almost all model-related tasks without my glasses.) It struck me that the lettering on that CG shield looked a little odd. Close inspection revealed that it read "Union Pacific." Oops.
Regarding additional photo-etched parts - I do think there's a small market for sailing ship parts. Several companies make square-mesh sheets that work for gratings, but finding them in a believable size is a hit-or-miss game. A series of grating sheets on various common scales, to be cut to size and shape by the modeler, would certainly be welcome. Generic netting, for such purposes as the bowsprit netting I mentioned earlier, also would be nice to have. On really small scales (1/350 and downward), photo-etched footropes, with built-in sag, might look ok. (They'd have to be custom-made for individual yards on specific ships, though.) One fairly straightforward subject for photo-etching that I personally would like to see is a set of lettering, to represent carved lettering on a ship's bow and/or stern. It would be particularly helpful to have such an alphabet in inclined lettering. If you prowl the railroad decal department of a good hobby shop you can occasionally find gold alphabets that lean to the right (for the starboard side of a ship's bow), but left-leaning alphabets (for the port side) seem to be almost unheard of.
The big limitation of photo-etching for sailing ships models, of course, is the limitation on thickness. To my eye, at least, photo-etched parts representing three-dimensional prototype parts (railings, shrouds, and ratlines, for example) look great on 1/700 scale and pretty good on 1/350. On larger scales than that, the "flatness" becomes problematic. A photo-etched guardrail or ladder on any scale bigger than that starts to look like a piece of tape with holes in it. My inclination is to think that, on the larger scales that most sailing ship modelers deal with, photo-etching is most effective to represent stuff that really is flat.
Those are just some random thoughts. The topic is certainly worth discussing; I imagine other Forum members will have some other ideas.