Awaresoft has developed a signature module for Aware IM that allows capturing signatures directly in applications and save the signature to a BO attribute and I guess the module can be used for you purpose (sketch) as well. The module is a simple module though and I don“t know if it would be sufficient. The supersignature.com "addon" looks like it might be a bit more advanced + can be used on multiple devices which the Awaresoft developed module for the time being cannot. The supersignature.com "way" seems to be more expensive though and takes more work I guess.
Here is a bit more info:
https://forum.awareim.com/d/6281>
Another thing I thought about which would possible be better for the emergency service. Use a "real" incident sketch application and add the sketch to a attribute for all users to see. I have used Smartdraw for sketching various things and the application is really outstanding and the incident sketch functionality in Smartdraw seems to be really good (havent used that particular part of the application but browsed it and played with it some).
http://www.smartdraw.com/
http://www.smartdraw.com/resources/glossary/