Thanks for this very helpful suggestion. Adding the option to the MCQ inputs for an "Other" input, using the existing algebraic input type, would be technically not all that difficult at all.
Indeed, the whole point of STACK MCQ types is that they already have to return a valid CAS object. The validation code is already there, and so I can't (on the face of it) see any insuperable technical difficulties. The real problem is providing the question author all the exiting input options for the algebraic input inside an MCQ...... I'll think about this.