Combined DHCP/PPTP for cable modem needed

Hi,

I checked all of the firmwares currently available here and on the official ASUS site.
I was wondering that nobody in this forum had talked about the following problem that I have with my infrastructure. Please help!

Environment: ASUS WL500g with TV cable modem (60MBit/s symmetric)

Problem: new operation mode for WL500g needed, as follows:
1.) WL500g gets dynamic IP from ISP's DHCP server (results in IP, netmask, default gw, DNS servers)
2.) WL500g performs PPTP on an IP address of the IP net got in item 1.) => results in another P-t-P IP address
3.) WL500g sets default router to P-t-P IP address discarding the previous one from 1.)

Does anybody knows about a firmware to setup this special environment?

Any help is highly appreciated, Thx!

I have an old version of oleg's firmeware patched to support pptp after dhcp. I'll check in the evening when I'm at home if I still have the sources.