According to the docs, it seems doable.
The request-promise docs says
Since request-promise wraps around request everything that works with request also works with request-promise. Also check out the request docs for more examples.
And the request docs says
httpSignature - options for the HTTP Signature Scheme using Joyent's library. The keyId and key properties must be specified. See the docs for other options.
I've tried adding a httpSignature: {key, keyId}
parameter to my request options, and it works, except it sends the private key along with the request, which can't be right.