If you want to remain a bit more standards-compliant, it sounds to me that what you'd want is wallet software which:
1. Creates multiple BIP-44 accounts from a single seed, and allows you to export the account extended private keys.
2. Allows the import of an individual account extended private key.
In practice, this doesn't get you very far today. The only wallet software I'm aware of which can easily do (1) is Mycelium for Android, and the only wallet software which can easily do (2) is Mycelium for Android and Electrum (on the plus side, the two are compatible—you can export an xprv from Mycelium into Electrum and result in the same keys/addresses).
If you'd like to stick with the Bitcoin Wallet for Android, I think what you're doing is about as good as you can get.
So I tried to write a new protobuf wallet file which contains:
- a fake deterministic seed
- 4 main keys (m, m/0', m/0'/0, m/0'/1')
which kind of does what I want: I derived the main private key
m from my SMpK, so it really is
m=
m/0' with
m being my SMpK. As I understand this, bitcoinj derives its keys from the 4 main keys.
I am still worried though, that at some point, bitcoinj could regenerate the wallet from the seed, which is just a placeholder now...
I'd be a bit worried too.
How does it respond to a protobuf with no deterministic seed at all (it's an optional protobuf record, after all)? That should be a bit safer if it works, yes?
Also, I wonder if you could "trick" Bitcoin Wallet into accepting a BIP-44 account? In other words, generate a BIP-44 account at m/44'/0'/#', and then create a protobuf with m/0' equal to the generated xprv, and also include m/0'/0 and m/0'/1'. Do not include either the deterministic seed nor the key at path m (just to make sure Bitcoinj doesn't overwrite m/0' with the wrong xprv).