In answer to the topic in question.....
Yes, the B737-800 can be forward slipped to increase descent rate on final approach but in 5000 hours of flying this aircraft I have only seen it done once and I am hoping that I never see it again. It is extremely uncomfortable - and not just for the passengers.
To end up in a situation where it would be verified is extremely unlikely and would be the result of incompetence on behalf of not only the ATC at the time but also both flight crewmembers.
Orbits on final have their own dangers (see the A320 crash in Bahrain whereby the circuit was performed with neither height loss or speed loss) however are alot more comfortable for aircraft and crew.
I am sure that Boeing recommendation for forward slipping as a way of increasing descent rate on finals would be the same as their recommendation for using speedbrake with flaps more than 15 or maintaining flap limit speed when extending flaps (using flaps as speedbrakes)...... i.e. NOT.
Do it once if you don't believe me. After doing so I am sure you will not do it again if your airline wants to keep their passengers.