Is it possible to use Eclipse variables like ${workspace_loc} or ${eclipse_home} for adding a JRE to Installed JREs in the Eclipse preferences?
No, you cannot use variables for that, neither in the JRE preferences nor the execution environment specifications. And don't look at the eclipse.ini as suggested in comments, the JVM running your IDE has nothing to do with the JRE used to compile your project.
That said, there is still another easy solution (which we use in our company): Just install a second copy of the end user JVM at a known fixed location for your developers, so you can use that fixed path in the JRE preference page.
If you fear that the two JRE copies (in your project and at the fixed location) get out of sync over time, then you could use an Ant based builder to your Eclipse project, which conditionally copies the JRE from the workspace to the outside known fixed location during the build. But honestly, it does not really matter if those 2 copies get out of sync. If your code was compiled using a 32 bit JRE, it will run on another 32 bit JRE deployed at your customer anyway, as long as they are byte code compatible. And you can set that explicitly in the Java compiler settings of your Java project.