Does Mogan require that these plugins be present to function even if the user will never want to open a maxima or octave session, and if not, how easy would it be to remove these plugins from a default installation?
Given that a recent survey on this forum revealed that Mathematica is by far the most used computer algebra system, it might make sense to include a plugin for the free (personal use) Wolfram Engine and/or for the free Mathics clone of Wolfram language as the ‘default’ CAS, or no plugin at all.
Developers of TeXmacs/Mogan may also want to take a look Wolframite to see how well its developers have integrated the Wolfram Engine with Clojure. Obviously Clojure is not Scheme, but the project shows what is possible.
WLJS Notebook might also be able to provide additional inspiration.