Sun filters were expected to be used with Huygenian eyepieces where the focal plane is actually inside the eyepiece and so the filter is far from it and probably not subject to great heat concentration.
In theory, perhaps, and an interesting point; yet, in the history of broken Sun filters, surely many broke while used with the Huygens sold with many classic telescopes. Using an eyepiece filter with a Huygens eyepiece places the filter within an inch or two of the most concentrated rays, exposing it to far more heat than, for example, a perfectly safe filter over the objective. One hopes readers would not misinterpret this as meaning it were okay to try using an aging eyepiece Sun filter with a Huygens eyepiece. Don't do it. Not worth the risk!