John This is my reasoning, and yes, the poor old breather does get the blame. Cam endfloat depends on the bushes, accuracy of casting machining plus individual parts tolerances.
The cam is supported on two identical top hat bushes, and a third bush in the drive side case. The cam can move sideways to the timing side, restrained by the top hat of the inner bush contacting the shoulder on the camshaft. Movement towards the drive side is restrained by the underside of the timing gear contacting the top hat of the outer camshaft bush. So, without the inner cover, breather sleeve and cork washer, the cam has some freedom of axial movement. This is prevented entirely by the compression force exerted by the cork washer, effectively eliminating any float by keeping the cam firmly pressed towards the drive side, the sideways load acting on the outer bush and underside of the timing gear.
The breather sleeve is effectively open at both ends. OK, there is a dividing wall in the middle, but this has a central hole, so you could say that crankcase fumes can enter from either end. The critical factor is not so much an airtight seal against the cam gear, more the necessity to ensure the holes in the breather always coincide with the hole in the timing cover as the breather sleeve rotates. This is done by compressing the cork washer, pushing the sleeve into place against the cover, preventing any sideways movement and effectively locates the breather sleeve and cam.
If my reasoning is correct the breather should work just as well with a compression spring substituted for the cork washer, pressing breather sleeve and cam towards their respective locations. The drawback here is that more oil rather than fumes could be drawn out, as the standard arrangement is a bit of a labyrinth to allow elimination of gas rather than liquid, so what we all consider to be an airtight seal is in fact more of an oil tight one.
Swarfy