I managed to compile openjfx on Trusty. No changed were required on th openjfx side, but I needed to backport gradle 1.5-1ubuntu1 from Utopic and werken.xpath 0.9.4-15 (the symlink-fix part could probably go in as an SRU in Ubuntu Precise and Trusty). After that, openjfx compiled, and I was able to install it. However, I found two issues:
1. For amd64 users, the jar files are installed in the i386 directory (since they are considered to be architecture-independent). However, trying to run javafxpackager from the command line (with and without arguments) results in a missing Main class. Symlinking these jar files over to the amd64 directory fixes this. In addition, this makes it a little easier for use in IDEs (at least in Netbeans).
2. After making the above fix, running javafxpackager results in an error about an unsupported major.minor class version. The shell script is trying to run java, and since the default Java is still Java 7 (while the jar files were compiled with Java 8), the error message occurs. Setting the JAVA_HOME environment variable fixes this.