I don't think most platonists would recognize the need for any relationship between actual, concrete objects and abstract objects, at least not in this direct way. It is useful, however, to distinguish between two types of platonists here (I take this classification from Sam Cowling's
Abstract Entities, a very useful---and opinionated---survey of the terrain).
Expansive platonists include among the existing abstract objects not only numbers, pure sets, and propositions, but also what has commonly been called
abstract types, such as, perhaps, musical works, poems, recipes, letters (in the sense of "a", "b", etc.), etc.
Austere platonists countenance only numbers, pure sets, and, perhaps, propositions and properties. So
expansive platonists may want a relation like "partake" between types and tokens, perhaps in the form of instantiation. But
austere platonists will not need any such a relation. If there is any relation between concrete "triangles" (I use scare quotes to indicate that I'm not taking such "triangles" to be literally triangles, i.e. instances of an abstract type) and abstract ones, it will probably be one of approximation (say, we can map an abstract triangle up to a margin of error to the space-time points occupied by the concrete triangle), where this relation is itself an abstract set.
As for your proposal, I think there are a couple of separate issues here. First, there is the question of whether there is a conceptual distinction to be made between abstract and concrete objects. Notice that this is a
conceptual question: perhaps there is such a distinction, but all objects fall only on one side of the divide (that is, e.g., the nominalist position), or perhaps the distinction is not exclusive, i.e. there are hybrids (which is what I mentioned with the problem of classifying {Nagase}). Still, it must be possible to make such a distinction independently of any ontological questions.
Second, there is the question of whether the ultimate constituents of reality fall on one side or the other of the division. Some people believe that reality is hierarchically structured, with metaphysical atoms at the bottom and every other object constructed out of such atoms (and constructs of such atoms) by way of metaphysical operations (perhaps "building" operations in the sense of Karen Bennett)---one model for this is a cumulative set-theoretical hierarchy with (or without) ur-elements, with the ur-elements (or the empty set) being the atoms and the "set-builder" operation being used to construct the other sets. Given this, it's possible to ask what reality is
really like, that is, how to characterize the objects at the bottom (the fundamental objects, those that
really, really exist): is it abstract or concrete?
Third, supposing the hierarchical structure picture is true, there is the question of the character of the constructed entities. Are they abstract, concrete, or hybrids? Notice that, if supposing there are hybrids, depending on the character of the building operations that you use in "constructing the universe", there may be a way to reduce everything to abstract entities. One influential program tried to do precisely this, supposing that everything could be constructed out of time-slices of atoms and sets of such time-slices (I think Quine held something like this). Here's the idea: pick any set of ur-elements; presumably, they are countable (if not, you'll need some choice to make the idea work). So map the ur-elements into the natural numbers and use this map to construct a set that is the extended image of the original set (more formally: if f is the original map, build f* recursively by setting f*(x)=f(x) if x is an ur-element, otherwise set f*(x)={f*(y) : y in x}). This will build an abstract replica of the original universe, preserving its structure. So everything you wanted to do with the original universe, you can do with the new universe. Simplicity considerations then may dictate that the new universe is the real universe. (I don't endorse this line of thinking, but it may be of interest to your program.)
Finally, about Shelah, my point is this: (A) some people define logic as xyz; (B) but xyz doesn't contemplate what logicians such as Shelah are doing; (C) therefore, xyz is not a good definition. I think this is true when you take "xyz" to be "the study of logical consequence relations", and doubly true when you take "xyz" to be "the study of relations between ideas". Personally, I'm attracted to a definition of logic as being the study of local invariant relations, but there is much here to work out...