The workaround you pointed to for single-column floats is a very crude one - the new environments simply don't float, which will almost invariably lead to bad column breaks without manual intervention. Footnotes are a kind of special floats, and an equivalent "workaround" would be to place text formatted akin to footnotes at the bottom of the affected columns by hand (and to determine column breaks by hand).
Quoting from p. 12 of the multicol documentation:
I'm not sure that I really want pagewide footnotes. But balancing of the last page can only be achieved with this approach or with a multi-path algorithm which is complicated and slow. But it's a challenge to everybody to prove me wrong! Another possibility is to reimplement a small part of the
fire_up procedure in TeX (the program). I think that this is the best solution if you are interested in complex page makeup, but it has the disadvantage that the resulting program cannot be called TeX thereafter.
So I'm afraid there's no workaround for your problem.