No, that's not possible. First, it is not possible to exchange custom property data without a custom form that contains the property definitions and is published to a location where both userse can access the published form definition. Furthermore, in the case of a meeting request, there is no mechanism to send custom property data back in an acceptance. The hidden form used for an acceptance is not customizable at all.
"Joeri Sebrechts" <joeri.sebrechts@gmail.com> wrote in message news:273404a7-79b4-448d-a993-f684189b5803@n58g2000hsf.googlegroups.com...
> Is the following scenario possible?
>
> 1. User A sends a meeting request to user B
> 2. User B adds a UserProperty to the generated AppoinmentItem
> 3. The added UserProperty is sent back to user A.
> 4. User A's AppointmentItem has the UserProperty.
>
> Currently this information is not sent back from the recipient, but I
> don't know whether this is just normal outlook behavior or something
> amiss in my code.