Do any hardware wallets / hardware platforms that rely on or allow open source firmware? There's a reason the safest software wallets are open-source: they can be audited by anyone. With hardware wallets, unless you're manufacturing one yourself, you're all but forced to trust the manufacturer to a certain extent. This trust could be minimized if a manufacturer created a generalized security device that users would upload their own firmware to. This way, the manufacturer would be very limited in the types of security holes it could intentionally add, given that they couldn't know what firmware would be running on it. 

So it seems to me that the ideal would be for a hardware wallet to have an open-source design, manufactured by some hopefully (but not necessarily) trustworthy private company, and each user would upload and run some open source firmware on the device. 

Are there any devices / hardware wallets that fit this description?
See here the latest list of 27 hardware wallets, many use open source software in the meantime - flagged with a (O). The other ones are flagged with a (N). 
For security reasons it is of course not sufficient to consider the software alone, also hardware can contain backdoors.

(O) Archos Safe-T (Mini)
(O) BC Vault
(O) Bitbox
(O) Bitfi
(O) BitLox Hardware Wallet
(?) Card Wallet
(O) Cobo Vault Wallet
(O) ColdCard
(N) CoolWallet S --> plans to (O) it in future
(N) D’CENT Hardware Wallet
(N) Ellipal
(N) Ellipal Titan
(O) Keepkey
(N) Keevo Wallet
(O) KeyCard
(N) Ledger Blue
(N) Ledger Nano S
(N) Ledger Nano X 
(N?) MIRKey by ellipticsecure
(O) Opendime
(N?) SafePal S1
(O) Secalot
(N) SecuX V20
(partly O) Temexe X Wallet --> backup algorithm is open source
(O) Trezor One Wallet
(O) Trezor T
(N?) XZEN Wallet