This is what I ended up with... I think it's kind of ugly, but it works 🙃
```
mod registers{
    pub struct PMSC;
    impl Into<u8> for PMSC{
        fn into(self) -> u8{0x00}
    }

    impl PMSC {
        ///PMSC Control Register 0
        pub const CTRL0: u8 = 0x00;
        ///PMSC Control Register 1
        pub const CTRL1: u8 = 0x04;
        //PMSC reserved area 1 
        //pub const RES1: u8 = 0x08;
        /// PMSC Snooze Time Register
        pub const SNOZT: u8 = 0x0C;
        //PMSC reserved area 2 
        //pub const RES2: u8 = 0x10;
        ///PMSC fine grain TX sequencing control 
        pub const TXFSEQ: u8 = 0x26;
        ///PMSC LED Control Register 
        pub const LEDC: u8 = 0x28;
    }

    pub struct OTP;
    impl Into<u8> for OTP{
        fn into(self) -> u8{0x2D}
    }
    impl OTP{
        ///OTP Write Data 
        pub const WDAT : u8 = 0x00;
        ///OTP Address
        pub const ADDR : u8 = 0x04;
        ///OTP Control
        pub const CTRL : u8 = 0x06;
        ///OTP Status 
        pub const STAT : u8 = 0x08;
        ///OTP Read Data 
        pub const RDAT : u8 = 0x0A;
        ///OTP SR Read Data
        pub const SRDAT: u8 = 0x0E;
        ///OTP Special Function 
        pub const SF   : u8 = 0x12;

    }
}
```

And how it's used:

l
```
et mut buf = [0x03, 0x01];
        Transaction::default()
            .set_write().set_register(registers::PMSC.into())
            .send(&mut self.spi, &mut buf).await?;

        buf = [0x80, 0x00];
        Transaction::default()
            .set_write().set_register(registers::OTP.into())
            .set_sub_index(registers::OTP::CTRL)
            .send(&mut self.spi, &mut buf).await?;
```

Is this a decent way to do it or bad practice?