Sorry, but could you give a bit more info about your goal here? Krausen is a very time-sensitive state, so I'm having trouble picturing what you're after.
Do you want to collect some yeasty wort and keep it handy for bottle/cask conditioning? That will be hard to control: too many variables. In general, I'd say that you're better off pitching a known quantity of healthy slurry with sugar, DME, or plain wort of a known gravity, which you can dial in more precisely with a bit of trial and error.