Which existing hardware wallets have expressed an interest in adding Monero support?
When will the Trezor code now being tested be made open source?
The Trezor code will be made open source when NoodleDoodle thinks it is ready. Currently it is only suitable for testing with very small amounts of Monero because until it is open-souce, it cannot be trustless. SatoshiLabs is willing to eventually update their firmware once the code is open-source and thoroughly tested. NoodleDoodle prefers to wait until after RingCT is released to open the source code designed to support it
It's just NoodleDoodle's work. I can ask if he'd like to accept donations for it. He said he doesn't want to release the source for the current version with RingCT coming so soon
AFAIK, there have not been any public reports of adding Monero support to KeepKey, Ledger, Case or any other hardware wallet. Most hardware wallet sales are made to Bitcoin users and Monero relies a completely different codebase. In the near term, Monero developers will likely need to write code to enable Monero support from additional hardware wallets. After that happens the companies creating those wallets may consider updating their firmware.