> I wonder if a compressed file system would be a better/easier/more
> flexible aproach

No doubt that compressed file system (or ffs with integrated compression) 
can achieve more compression ratio and can be more efficient.  But I 
doubt it is easier.  I consider high quality implementation of compressed 
filesystem an unmanagable task for myself currently.

> I'm not sure compressed swap is worth all the trouble to do it at
> the block device layer.

I doubt it is wise to allow swap to reside on compressed device..  
Compressed device has nothing to do aside from returning EIO when 
attempting to write block and having underlying device full.

Probably such device can be safely used only in read-only mode.  If you 
write to this device then if you are low of memory you are risking to 
receive EIO..