If the application that calls your OmniHelp project can execute system commands, the developer can have the application access the Windows Registry for the required browser command syntax, and use that command to open an OmniHelp topic file. With this method, you do not need redirect pages (see §19.11.3 Using redirect pages for OmniHelp CSH calls).
To see what is involved, check the Windows Registry for the correct browser command syntax (Start > Run > regedit):
and look at the first (Default) entry in the Data column. For example, for Firefox the default browser name is FirefoxHTML.
HKEY_CLASSES_ROOT\DefaultBrowserName\shell\open\command
In each call, the application should replace %1 in the browser command with the following type of file reference:
file:///path/to/_myproj.htm#IDH_contextID
where IDH_contextID is the content of a TopicAlias PI marker in DITA XML. Notice the three forward slashes in the file reference. This syntax should open the correct OmniHelp topic file.