In this paper about the SABRE engine, there is the following paragraph (emphasis mine):
The water vapour in the atmosphere up to an altitude of around 12 km is a problem for precooled engines, causing them to block with frost in a matter of seconds. A great deal of this water is precipitated in the liquid phase during the cooling of the air and has to be rejected from the engine before it can freeze. Provision then has to be made to stop the build up of ice within the matrix as it precipitates directly from the vapour. A major part of the experimental program has been to demonstrate that this can be achieved.
How might this be achieved? If there is more than one possibility, is there a most likely or an easiest way of doing this?
This should be seen as more of a brainstorming question than as an opinion-based one. After all, Reaction Engines was most secretive about the mechanism, so there will probably not be a definitive, public answer any time before 2013.