Could you say a little more about what you're trying to do? The exact thing you asked for - set a property when a user selects a feature for install - is not directly possible. But depending on what your needs are, there's probably a way to satisfy them.
If it's set a property when the feature will be installed (i.e. it's okay if it happens after the initial selection), you could schedule a Set Property custom action (or even a Set Property Control Event on a Dialog) and condition it against the states of the feature (see Conditional Statement Syntax in the help). If it's important that it only happens during an install with UI, also condition against the UILevel property, or use the Control Event which implicitly requires the UI.
If it's important that the user explicitly selected it, you may need to set up intricate conditions and extra properties to note that the user saw the feature selection dialog. Then as far as what the actual property and why you're setting it, if it's just to condition something else on the feature getting installed, it may be easier to short circuit and just condition on the feature directly like in first Set Property custom action I alluded to.
I currently have an .msi installer that works for a user to specified cmd-line properties to control installation parameters. The idea is that the installer would run from the command line using msiexec and run without a user interface (/qn).
Now there is a need to support the UI install and I was hoping to be able to set the same properties in the UI feature selection as if they were passed on the command line.
