Our product deploys via msi at clients' sites, and is an add-on to a sister product.
I need to detect the install folder of the sister product at install time, and install our product to the same location.
Checkout this blog article I wrote:
Redemption of Visual Studio Deployment Projects
This will allow you to "twin" your sister product to the naked eye but under the covers get all the advantages of a WiX project. This includes full use of custom search extensions and custom action scheduling.