Is it worth putting the whole inlet geometry problem into perspective?
Some factors:
1) This is a 100 knot airplane, dynamic pressure under standard conditions at that speed will be about 0.24 psi (1640 Pa). These Rotax heat exchangers have a net area of about 70 SQ inches, so even if we cut the inlet area by 50% with a near optimum inlet, the drag reduction would be about 8 lbs compared to just having the radiator face at/near the front of the duct.
2) In actuality, the difference won't be that much, because there will be at least some pressure recovery on the back of the duct.
3) Also, if we have (virtually) no inlet, and because the coolant HX and oil HX "stack" will provide resistance to airflow, any air that doesn't go through the exchanger will spill over the top/sides of the duct. With a well designed opening (well rounded edges, etc) this occurs with relatively little drag even compared to a solid "nose cone". This is one reason that modulating the flow of air through a coolant duct is usually done at the exit rather than at the inlet. Drag due to a "can't take any more" duct or a flat surface isn't very high at all IF turbulence is decreased with well designed 'lips" on the duct.
Bottom line: In this case, I suspect a "zero length' diffuser will generate >much< less than the "worst case" of 8 lbs of drag difference noted above. I suspect, at any rate, that the biggest contribution of the nacelle will come from cleaning up the airflow around and behind the engine.
All this aside, if an inlet over the existing BRS is still desired, I'd forget any idea of a sliding snout removal, etc. There are just too many ways for that to fail. I'd go with a single frangible sheet at the top of the duct as far away from the rocket as possible (to let it gain some kinetic energy before striking the surface). I'd test candidate panels with a rig that dropped a dull slug of the same mass as the rocket into the sample sheet at the expected V of the rocket at that point. And do it at different temperatures to assure it cracks as well at 100F as at 20F.
If a frangible sheet, pre-scored snout, sliding snout, etc departs the aircraft in normal flight, there's a good chance it will destroy the prop, at a minimum. Again, my choice would be to just have a minimal inlet right at the radiator face that won't interfere with the already engineered BRS.