I'm not 100% sure how this API works, but it looks like it lets you supply the entropy. Test to see if you always get the same address and secret if you supply the same entropy. If you do, then just use the mnemonic and any HD scheme you want to derive a secret of any kind (you can use the same algorithm you'd use for bitcoin) and pass that secret as the entropy to that API call.
Two cautions with this approach:
1) You must test to make sure that API really is deterministic. Try it on a few different machines and a few different JS implementations and make sure the same entropy always leads to the same address and secret.
2) Your ability to recover keys from the mnemonic would be tied to your access to this library. If the library ever changed its key generation algorithm, you might be unable to recover the keys. So pick a version that works and don't ever change it.