Nigel, there is a mode of communications used by ham radio operators that sends a puny 31.5 bits per second of data. You can fit 25 to 50 of them into one HF speech frequency allocation. And you can drag out 100% copy from signals you cannot hear with your ears in a normal SSB radio. (I have used it in the past to communicate with places like South Africa, Chile, and Europe from Southern California well after the 14 MHz ham band was close tighter than a drum for most other modes of communications.) I'd not worry about "too slow" as 63 bits per second and 128 bits per second variants exist. The latter would cut you down to 6 to 12 sub-channels per channel. An unencoded ACARS message would go through several times in 10 seconds or so on one subchannel. If planes are assigned "time slots" accommodating large numbers of planes would not be hard.
Now, to what benefit. It's not going to STOP a crash. It only tells you within 80 miles or so where the plane stopped transmitting. That's a LARGE area to search. So you've not bought much even for the postmortem search effort. And if the search area is socked in, as in this case, you still must patiently wait until you can get calm enough water that mostly submerged objects stand out to you when searching a thousand square miles or so.
(And the first time a massive search effort is triggered by a part in an ACARS radio gasping its last breath and shutting the radio down at the same time DAKAR is being derelict, again, the whole thing will be shut down.)
It might be more salubrious to lean on outfits like DAKAR and to a lesser extent ATLANTICO to invest a few more brain cells in their day to day operations.