This has been a very popular feature request that will be
productized, I don’t have time line for availability at this time but know it’s
coming. If you can’t wait for this I would suggest having a look at “Creating a
custom control” under documentation on http://help.k2.com this should get you
going with creating a custom control that will get you the required
functionality

Has any one out there done this?  A concrete example would be great!

In case anyone else comes across this post, this functionality has been added to the product as of SmartForms 1.0.2.

Video demo: http://help.k2.com/videos/6502

Help file content: http://help.k2.com/helppages/K2smartforms1.02/webframe.html?pdf_control.html