According to this topic http://forum.mikrotik.com/t/pppoe-dynamic-interfaces-and-upnp/6554/1 and still no success again i started same topic . this topic could be also in scripting forum but this also can be a feature of the next releases of mikrotik core . something like change tcp mss for pppoe users.
i know that in mikrotik v3.x we can do this work with an external programming language in conjuction of API but there is a limitation in v3.x for upnp http://forum.mikrotik.com/t/really-disappointed-from-upnp/22527/1 in addition to other problems of v3 
so it is better to create a script that do the job or even embed it in core.
i think if mikrotik has a feature named “on_connect” and “on_disconnect” (something like javascript event On_Load ) which can be used in scripts , these types of problems will be solved.
else a script that do the job in intervals of time should be written which is not good idea.