Today I found out that it's possible to apply a MasterPage both on Application Level and on Folder Level.
Application Level
<configuration>
<system.web>
<pages masterPageFile="~/mhaMP.master" />
</system.web>
</configuration>
All .aspx pages will now the use the MasterPage called "mhaMP.master" (if the .aspx file does not contain a 'Content' control, the MasterPage will not be applied to it).
Folder Level
<configuration>
<location path="admin">
<system.web>
<pages masterPageFile="~/mhaMP.master" />
</system.web>
</location>
</configuration>
Using a web.config file in the "admin" folder, which contains the above, all .aspx pages in the "admin" folder will now the use the MasterPage called "mhaMP.master" (again, if the .aspx file does not contain a 'Content' control, the MasterPage will not be applied to it).