I do not have such a solution built yet, but business to business is space that StellarGuard is likely to move into in the near future (currently it is just a consumer product).
Is this something you'd be interested in hearing about, or participating in an alpha as it's being developed? The brief overview of it would be that you'd add additional signers to your account (the StellarGuard signer) and then an API to submit and authorize transactions so they are signed by the StellarGuard keys. There would be access controls/limits/audit-able and revokable API keys, with access control and IAM options.
If you don't mind, I have a few questions:
1) Would you be comfortable with a solution that does not use HSM, but instead uses an encryption/decryption using Google KMS? https://cloud.google.com/kms/. Or is using HSM an requirement.
2) Would you use it if this was a Software as a Service model and not something you host on-prem?
Paul from StellarGuard