I've just had another thought!
If you're inserting tee's into the existing pipes, you'll have to cut a short section of pipe out. Suppose you get a couple of those self-piercing washing machine taps (can you get them for 22mm pipe if necessary?) and install them temporarily where the tee's will go.
You can then use the taps to drain the pipes - under controlled conditions - into shallow trays. Finally, when no more water comes out, remove the taps and cut out the pierced section of pipe.
How does that grab you?