Do any hardware wallets / hardware platforms that rely on or allow open source firmware?
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