Studio supports a number of built-in functions (the one I wanted was RANDBETWEEN()). However, if you try using these you get errors like "The method RANDBETWEEN(int, int) is undefined for the type ...". This is because these functions are contained in the jasperreports-functions jar which we do not ship in the OpenVPMS lib.
This is only a small file (32K) and should be included in the distribution.
The work-around is to download https://jaspersoft.artifactoryonline.com/jaspersoft/jr-ce-releases/net/sf/jasperreports/jasperreports-functions/6.2.0/jasperreports-functions-6.2.0.jar and install it in <TOMCAT-HOME>\webapps\openvpms\WEB-INF\lib