Dear Taco,
do you really need an electronique signature for your use case ?
An electronique signature is associated with a document: it will certify that the signee (certificate owner) saw the document (and the document was not tampered) and agree witht the content. The value of eletronique signature will highly rely on the agreement level of the provider and the legal surrounding framework (eg. in Europe you may need a SSD to make it legally binding - TBC ).
The problem is that with standard electronique signature you will need to have a certificate provided by a trusted identity provider and at enrolment time you should link the user/password with the certificate.
Maybe you are more looking into strong authentication mechanism (that can also be based on certificate), currently NW SSO provide you some services in this respect:
you may check : (http://scn.sap.com/community/sso/blog/2014/05/12/stronger-authentication-with-one-time-password-solution) or check the new SAP Cloud Identity Service (https://help.hana.ondemand.com/cloud_identity/frameset.htm)
I hope this help,
Best Regards,
jc