Hi littleshobo,
Rel=author seems like it would be more beneficial to you in your personal guest-blogging than, as you had said, to attach your face to your clients' blog posts. You do want them to be attached to someone at the business. A different story might be if you are writing content that indirectly benefits the client: Say you guest blog on an external website about something you're interested in and you link to your client's blog as an example of your work. In that case, you'd want your face to be attached to that blog.
I would try to keep the client's blog attached to the client.
I am going through the same thing right now. What we are implementing is the rel=publisher tag on our clients' websites, using the clients' Google+ Profile ID. Though it's not being recognized right now by Google, we hope in the future Google will begin using it and show the dealership's logo in the rich snippets.
The other option for you is to find a person at the company who wouldn't mind their face being the face of the company, and you could "ghostwrite" for that person. You could then use the rel=author tag for that person and explain to them how they would need to add the corresponding link to his or her Google+ profile. The only issue with this is trying to find someone who would be at the company for awhile (avoid turnover) and also someone who would enjoy having their face plastered across Google. Some higher levels in the company might be okay with this.
Also, check out this link about using rel=publisher and rel=author from Mike Blumenthal's blog. http://blumenthals.com/blog/2013/02/03/google-local-rel-publisher-or-relauthor-you-should-do-both/
Hope this helps!