Id be very dubious about applying slip if the standard operating manual doesnt mention it.... Its a bloody good trick to get your sailplane on the deck ASAP, however, PAX generally get pi**ed off when they have to lean sideways to drink their G and T.
Reasons behind flap limitation to aircraft sideslip occur as a result of decrease in control surface authority (horiz. Tailplane mainly) and even stall of the rudder. Not Healthy. Buffet Generated by high alpha manoevers on flexible tailplanes can significantly decrease the fatigue lives of their constituent components, as per tornado GR1, B707 F18, F16 etc etc...
As regards buffet entering engine intakes, its probably studied in the design process, however, having a background investigating Resonance induced blade failures and its complexities, it would definately be worth asking the chaps at boeing.... I cant imagine them looking at it in too great detail, pilots are supposed to stick to straight and level as much as possible to keep the boffins happy.Could always try it in the sim, or just ask your fleet chief instructor...
From an engineering standpoint, its probably best to avoid, as you are entering flight envelopes that are cleared for flight, but not as standard procedure and thus can have no idea as to the long term effects.
Hope this was helpful