I don't know much about polymer chemistry, but I imagine this is going to be very difficult indeed. Which polyether are you working with?
In a polyether functionalised with methoxy residues, there is only one functional group present in the whole polymer - ether linkages. While the methyl ethers will be more susceptible to HI/TMSI/BBr3 etc.-type procedures, all ethers are vulnerable under these conditions and I suspect that getting decent selectivity will be tough.