Phil - Recently did a mod for a customer who used to have a POS system.
In SO Invoice Data Entry on Totals tab for their retail counters they wanted a button that would integrate to a Topaz signature pad. The customer would sign the sig pad at the counter for their purchase (http://goo.gl/Lwx51h). After signing, counter clerk presses a Capture button on Totals tab to capture the signature, store it on a network share, save the link to the signature image in the invoice header, and finally print the sig on the printed invoice. I know not exactly what you're looking for but had to mention. It was pretty killer :)