Wix: Custom Action to call batch file and set launch condition

I've got a Wix situation that seems like its solution might be the combination of several steps. I just don't know what the steps are or how to contruct them.

My installation depends on a certain version of another program being install (e.g. OtherApp v2.1.6). I need to specify that as a Launch condition.

The tricky part is that the only place that version is specified in the OtherApp installation is in a textfile (version.dat). So I created a batch file to open that textfile and compare the version found with one that is passed into the batch file as an argument. The batch file returns 0 if the version is the same or higher (assuming backward compatibility), and 1 if the version is older.

I've already tried to create a Custom Action to execute the batch file (which I'm not sure I'm doing correctly either), but I then need to somehow set the Launch condition based on the return of the batch file (ERRORLEVEL).

Can anyone give any insight as to how I should set up this scenario? I thought I knew, but I've found that I haven't a clue.

Thanks for any help!