Anyone know of any obvious reasons why this Idea may not work?

I'm hoping it would be an easy swap, Fog light out / DRL in, splice a few wires and just use the fog light switch to switch them on!?

In the UK you get fined for 'inproper use of fog lights', basically not using them when its foggy! And contrary to popular belief, its not often foggy here

Would rather the DRL's

 

Unless the Euro market GS's are different the DRL is simply the high beam bulbs being fed a reduced voltage. This is controlled by a circuit somewhere and does not use a separate wire that can be cut and rerouted to a different bulb.

I looked long and hard at this using all my resources for wiring diagrams and such and decided even if possible it would probably be more work then it is worth. That said you may find a way I did not think of at the time. Good luck if you go for it.