Ok, strange title I hear you say.
Now that I've got your attention, I need your help.

I've got an 1988 N/A RX7, no LSD.
Recently, while in the wet, say if I go round a corner and get it sideways (easy with no LSD), if wheelspin is sustained for more that 3+ seconds, there will be a sound, which sounds like driving on water (whooshing?) but a lot louder coming from the rear. What could this be?

And, after wheelspin, the brakes are virtually non existent for quite a while. Any suggestions as to what this might be?

It isn't thrashing either, tacho will rarely see over 4000rpm.

 

Sounds like you may have a sticking/stuck rear caliper.

But based on your description it's just a guess.

 

Thanks for the suggestion, it's an odd problem so any suggestion is good!

If a rear caliper was getting stuck, how would I go about unsticking it, or even checking if it was getting stuck?

 

Take caliper off and inspect/clean/lubricate the pin/bolts that the caliper floats on. It's not hard, just don't expect to wiz through it in 5 minutes.

 

probably worth my while buying some second hand ones as my front one's grind badly anyway

 

if a caliper is sticking you might see excessive brake dust on that wheel.

 

And heat on the wheel as well.

I've found that once a caliper starts sticking, the only real cure is to replace it.