It's an interesting idea and I can see the point, but adopting new forms is a difficult process. I once tried to use American spelling for a week on a forum and abandoned it very quickly.
I have no problems with using the plural- it works in other languages, as seen in T&V forms in French, etc. I see the politeness principle as one that can over-ride numerical accuracy. The concept of numerical accuracy of the singular is also not always true:
Someone hus umbrella.
It could be that a single person or a couple left it. Wouldn't it make sense as a non-number specific word, too? I'll try to use it over the next week and see how it goes.
