Ok, so now that I've solved my problem I'm thinking about how this can be applied to your problem.
You could include the host name in the alert file name your rule logs to, using the %MsgHost variable. This would create a separate alert log file for each host. The log file name would be something like:
C:\Batches\Rule1 %MsgHost.txt
The batch file would then look something like this:
if not exist "C:\Batches\Rule1 Host1.txt" GOTO End
Note that I have changed this to the file and path in quotes because the file name will not conform to the 8.3 file format, a holdover from DOS.
Unfortunately, it would require having a separate batch file for each host so that it would continue to email for each host, and each batch file called by Windows Task Manager every half hour. Only the file that the batch checks for would be different, and possibly the subject line including the host name, so it would be simple to do but very repetitive for 100 hosts. It also means that every time a new host is added you would have to create another batch file for the corresponding host name.