The device itself would mass around 20kg (50lb) and could be mounted below the aircraft - that's achievable. Certainly they exist for much higher inertia aircraft than an R22. Costwise, you're certainly looking below £6k/$10k, which isn't a great deal compared to the cost of any helicopter.
Operating it could be solved in a similar way to the Yak-38 "Forger", a Soviet harrier copy that sat on a tripod of three gas-turbine engines. If you lost a single engine (I'm not sure how this was sensed, but I'd guess a comparison of N1 values combined with a weight on wheels switch) the ejection was automatic. I believe that the red-navy's record was that about 2/3 of their (fairly large) number of Yak-38 ejections were automatic. (Must be a bit of a shock to the system, but better than the alternative).
In a helicopter, an RPM drop sensor combined with a weight-on-skids switch would probably do the job and be fairly easy to mechanise.
A much more difficult problem may well be the integrity of the suspension loom. It's fairly common in FW applications to mount the parachute and it's ballistic drogue in a canister below the belly of the aircraft. But, then the suspension loom needs to be routed round the side of the aircraft to a suitable attachment - in an R22 I'd venture that the gearbox assembly would probably be that. So, the chute fires out the side, swings round the top, and the aircraft then descends to impact on the wheels or skids - the only bit of structure designed to take a hard ground impact without killing the occupants. The fact is however, that this loom will have to be man-enough to take impact with the rotor without being weakened sufficiently that it breaks under the shock load of a falling aircraft at one end and an opening parachute canopy at the other, or without being wrapped into a still rotating rotor and bringing the canopy in with it. There are looms available that will go through a rotating propeller and survive - but a helicopter rotor is a much more daunting prospect and this would be an "interesting" engineering problem to solve.
So, I think (and incidentally, I'm the bloke who wrote the UK's civil rules for fitting these things onto FW aeroplanes) it's possible.
The other question is - is there any point? As has been pointed out, a competent helicopter pilot should be able to auto to the ground anyhow. If the aircraft is routinely operating over terrain where this isn't feasible - say forrested mountainside, shouldn't they be operating a multi-engine helicopter anyhow? Of course it would save your bacon in the event of a structural failure of, say, a main rotorblade - but is that a realistic risk that needs to be guarded against?
In the fixed wing world, there are two main environments that they are used in - the Cirrus which has never been shown to recover from a spin, and in German where it's mandated for microlights (but their structural rules are such that structural failures are I'm afraid quite common). Most of the rest of the world either has high enough structural standards, or insists upon a spinning evaluation, or both. I'd regard any suggestion of fitting ballistic parachutes to helicopters in that light - where's the need?
That said, they have their uses. In light fixed wing flight testing (my mastermind specialist subject) it's an elegant safety solution for high risk trials such as stalling or spinning of a new prototype. But the occasions where they are really a good idea are few and far between.
G