FlexBuffers could support a similar scheme if we wanted to, though putting custom data in the format is already easy enough using the "blob" type, which is arbitrary bytes much like the MessagePack feature.
Sure, it works via stuffing the custom things in blobs, but then you have (1) an extra layer of indirection during dispatch, and (2) always waste a few bits or bytes to store the custom type information, even when you don't need it for actual blobs.
In principle, it's not a lot of effort to reserve a fraction of the type ID space for custom types. From a user's perspective, it's also much nicer to hear that a library cleanly uses existing extensions to the type system, rather than hijacking a general-purpose blob type.
If there's nothing that prevents it, feel free to take this as a feature request :-)
So this feature would look like a new function IsCustom1() or AsCustom1() etc, where the latter would give you a pointer to the bytes to do your own thing.. so not much difference with Blobs.
Also note that because FlexBuffers is O(1) access to elements, inline data (most scalars) is all the same size, whereas variable sized data is stored over an offset. So again, would not be very different from blobs.