Nobody really knows at this stage though it would seem it may be easier to do the conversion the other way around. Assuming the PAL Gamecube supports a 60 hz mode then it would only be a case of defeating the region lockout. This may be difficult or may be easy but if Nintendo stay true to form, it shouldn't pose much of a problem. Nintendo are not as concerned over piracy as Microsoft and Sony as their Gamecube discs use proprietary technology, which is one the main reasons why their Jap/US region mod was a simple system board jumper.
The problem with the JAP/US machines playing PAL games is the 50hz issue. In the past Nintendo have used a region lockout mechanism that matches the software speed to the hardware speed so even when the region lockout is disabled, import games will still not run unless the hardware is running in at the same hz rate. Therefore it may be necessary to implement a 50hz switch in JAP/US Gamecubes in order to play PAL games.
Rest assured, there are many of us that will be working on this as soon as Pal hardware is available. Needless to say, the first port of investigation will be the same Jumper block used in the JAP/US mod, then a thorough investigation of the RGB output of the PAL console to see if it can be implemented in the US/JAP machines.