Hi,
I notice you have Amazon S3 support now for WinSCP – any chance of getting Joyent's Manta storage API supported?
https://apidocs.tritondatacenter.com/manta/
I've written some basic client stuff in PHP for Manta before – the only really tricky aspect of the whole thing is the URL Signature scheme, but once you understand how it works it's really basic to operate.
The login dialogue would require four things – base URL (similar to S3's "Host name" field), username, public key fingerprint (16 bytes represented as colon-separated doublehex numbers), and private key file (guess you could specify a file on disk, enter it as text, or possibly even hook pagent or something).
Cheers,
Jon.