I'm not sure if this is still an issue, but in my quest to learn InfoPath I found this article which really helped on Digital Signatures:
http://www.thorprojects.com/blog/archive/2007/06/30/domain-certificate-authority-signing-infopath-2007-forms.aspx
And it did just as you were mentioning, created the signature as a line to click on, pulled from a Certificate Server.