I run Bitcoin-QT on an Ubuntu that's installed to a small SSD, & the blocks folder is on a larger HDD.
It's blocks folder is over 145GB & I'd like to be able to reduce that somehow. At first I looked into there being a way to compress individual folders like Windows can on NTFS, but brief Googling told me that's not a thing in linux.
Any tricks to do this?
There is no real way to efficiently compress it and used it as such, since if you do compress it, you will be spending some CPU power whenever you need to access it anyway, but the previous reply is also right, it won't compress much.
Are you asking this so you can fit the blockchain on the SSD?
If so, you could try instead of pruning it, to maybe put the older blocks on the HDD and newer ones on the SSD. That should speed things up for the most requested blocks. You could probably do this by making soft (symbolic) links of the certain amount of oldest blocks until you are comfortable with the amount of free space you have left on the SSD.
You can see more about creating symbolic links here
https://en.wikipedia.org/wiki/Symbolic_link#POSIX_and_Unix-like_operating_systems