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Is a high alpha sideslip possible and safe?
We use a sideslip to increase drag when trying to reduce altitude before landing on a short runway, etc., which is crossing your rudder and ailerons, to produce an angle to the intended direction, usually in level flight.
Is it possible to do a sideslip at an AOA of say 15 deg for even more drag, assuming stall angle is at 30 deg?
You would be risking your life. Cross controlled approaches should only be done well above stall speed and only after testing at a higher altitude. Increasing AOA will slow your plane down. If you recognize being high early in the pattern, you might be better off going ABOVE Vbg and slip, which makes the plane glide much less efficiently. Spoilers act in a similar manner.
Do a proper approach and use your flaps/spoilers. If it is allowed, forward slip to increase rate of descent at your preferred approach speed. If you can't make the runway GO AROUND. Changing speeds on short final will make judging the landing much more difficult.
In an emergency landing it is far better to over shoot the field and roll into fence, trees, etc at 10-20 mph than to risk losing control of the plane.
As a side note, only deltas come close to 30 degrees AOA before stalling, and these tend to drop like a rock when slipped if their vortex lift is sufficiently disturbed. Slats and flaps will significantly increase stall AOA, but at the expense of much more drag.
Generally a bad idea.
Let's assume that to first approximation, a given rudder deflection (say, full deflection) creates a given slip angle of the fuselage. So for a given rudder deflection, don't you generate a lot more drag (and also sideforce- thus requiring a steeper bank angle if you wish to maintain a linear flight path) if you are flying at a HIGH airspeed than at a LOW airspeed? So, there may be a wrong premise imbedded in this question.
Yes it's safe as long as you know what you are doing and there is adequate alpha margin while slipping, and you are adept at managing the energy of the very high sink rate that results. If you were approaching in something like a 701 at, say, 35 mph, and you wanted to slip to increase the descent rate, you could do so but it would be prudent to reduce alpha somewhat and add energy by speeding up a few mph while in the slip. So you would do it like any other airplane, kick in the slip and lower the nose a bit at the same time, maintaining ample margin above stall. So if the stalling AOA is 25-30 deg and you are at 15 at your normal STOL approach speed, not a problem.
The main thing is, in a STOL airplane with very high drag in the landing configuration, you have to make sure you have energy to arrest the sink rate close to the ground by having excess inertia energy and being ready and aggressive with power to control the last bit before touchdown. The wing may not stall until it's at 30 deg, but when it's getting up there the drag is massive and the increase in lift with extra pitch declines and you can find yourself dropping out and landing hard, even though you are not actually stalled.
