Hello Mike and MMD.  I've been using Tyvek for several years to
cover the bellows on cuckoo clocks, and for that use, it is perfect.
It lasts for many years, although I have no way of knowing how long
it will last.

That said, I must reiterate what has been said by others; that
traditional materials which are time-proven are the things to use.
In the above-mentioned clocks, I'm careful to use Tyvek only in clocks
which originally had Tyvek or paper bellows (this latter includes most
post-WW2 Japanese cuckoos).  If the originals used leather, then
I replace them with leather.

For something that has a hard working life, such as the stack in a
player piano, I don't think the Tyvek will hold up.  Remember that the
typical cuckoo bellows/pipe combination works on an pressure of about
3/4" wind to around 2" wind.  The vacuum of a piano (20" to 40") might
suck the Tyvek right off!  Things to ponder...

Take care!
Bob Loesch - formerly Cuckoobob's Clocks
Northern California