Might try using some citric acid. They oldsters used to use lemon juice to get rust and ink out of clothes. They're both porus so should work the same. BUT! I'd do a little experimenting on some broken pieces to get your system down before going whole hog on a good piece. You can get citric from home brewing suppliers.
Are those all from one privy or the back of one pub? Excellent display you've got! I'd do some checking before doing a lot of restoration on them, might drop the value. I know merscham pipes that have been used enough to get the browning from use command a higher price than new ones or ones that have been bleached. Let me know which technique you decide on & I'd be interested in seeing how they came out.