Actually I decided to go the way Spark sent me to. I think to just add a pool for 3% of the traffic might be the way which costs the less performance. And performance is a high good to us.
Although, there is a little glitch. I can't just use a dummy-pool, but would add a real pool, as the subdomain I want to redirect to, would use these servers.
But as far I understand our f5:
As soon as the f5 took the decision to direct the traffic to a certain pool, the iRules have been passed already.
Or is there a way to interfere with iRules at this level?