I have some further good news!
Thinking on it more, Dialog isn’t really that inefficient about per-object variables. Inform 6 has several properties that apply to all objects that Dialog doesn’t, like the dictionary words, short name, and description. So why is it such a problem to use per-object variables in Dialog?
Well, with this bug fixed, it seems like it isn’t! I added twenty extra per-object variables to Miss Gosling and it caused no problems whatsoever on a full-game runthrough. I think we can officially stop telling people that they need to avoid those!