indulis - that is far from clear in the original post. In fact he says "I also have a wireless router" which suggested to me it was a separate device, and that is how I answered. But if it is the same device, then that can be left on and PC attach to it, and it provide DHCP services exactly as it currently does, without effecting LMCE at all. You have to accept that in this configuration, these devices will not be able to interact with LMCE without making config changes. One of which, you are right, would be to setup specific rules on the firewall to allow access (turning it off is highly inadvisable as this disables the "NATing" functionality as well, which can be very useful/essential for inbound connections). Also, pnp will not work, etc.
In short, you would have to do some re-engineering of the environment which is likely to cause you headaches later. My advice (obviously!) would be to do what I did in the same situation - I just disabled the wireless on my ADSL router and bought a cheap AP and placed it on the internal network. For the sake of headaches, loss of functionality and literally a handful of dollars, it was the best decision I ever made in LMCE-land!
To answer your question, on splitting out the wireless functionality - no, realistically I don't think it will be possible. I have a high(est) end Billion modem and I went into some considerable detail with their support to work out if I could use the VLAN'ing functionality and port mapping to separate out the wireless LAN interface and place that on the internal network. The answer was "no". You can do something like this with the wired ports, but the wireless interface is permanently bridged onto the same switch domain. I suspect that this highly specific configuration is beyond most ADSL modems.
I think, by far, my solution of a one-off payment of 20-30 bucks for an AP provides the most flexible, simple and uncompromised solution - its a matter of biting the bullet! But you do have the option, if you are masochistic enough, you can work around... your choice