There are many possible ways to handle this.
The important concept to remember is that the first master file found in the application path that matches the name will be used. As a result you can use APP PATH, APP PREPEND, etc to adjust the APP PATH to get the desired result.
With that said:
1) Server profiles - If the users are going to be different between ReportCaster and Developer Studio you can use user or group profiles to adjust the application path.
2) Site Profile - You can use WFS Scripting logic within your site.wfs to add different entries to the _site_profile based on whether the request comes from Developer Studio or not to adjust the application path.
(See my 2010 Summit Lab
for an example of setting _site_profile. The appendix at the end shows how to identify DevStudio)
3) As Lidholm suggests you can use different Reporting Server's that have different application paths set with them (e.g. The local DevStudio Reporting Server, and the external server).
WF 71.x, 76.x, 7701, 8.0 Beta OS: Linux, Win2k3, Win2k, Win2k8, WinXP