Its basically a matter of people having the time.
The most recent kernel is 2.4.18. I did get a 2.4.24 version to compile, but it had problems starting init and I haven't had time to investigate.
Assuming there is still support for the older ARM processors/systems in 2.6, the only thing keeping the webpal from running on it is somone doing the work. I haven't investigated this fully to say more. You would want to look for arm "rpc" and/or "ps7500" support. The more those systems/processors are supported, the easier it will be to get the webpal to work.