Thank you for your post.
At this time, after you tap a Container field, the option "Get Signature" will only take you to a new screen where you can enter the signature. You cannot have the signature appear over an existing layout.
I encourage you to enter this suggestion into our Feature Requests web form at:
The entries in this web form are put into a database file that is monitored and read by Development and Product Management, where they are then discussed and considered for a future release. Although I could copy your post and paste it into the web form, there are a couple of additional questions asked that only you can answer.