Turns out this is actually possible! I was making my own little texture inspired by this (just adding roots to nether fungi, not planning on uploading it anywhere), and I saw that in the Optifine documentation they listed stair orientation as an example, which made me curious. I checked with F3 in-game and found that the log orientations are just worded differently (most other directional blocks have 'facing: [​south/east]'). For logs, basalt, chains etc, their orientation property is 'axis: [​x/y/z]'.

So, if in later versions of this texture pack, you only want the roots to apply to vertical logs, you could have the properties file say:
connectBlocks=[wood type]_log:axis=y
If anybody doesn't want roots on horizontal logs and they feel confident enough with it, just open up the '.properties' files using something like Notepad++ and chuck ':axis=y' on the end of the 'connectBlocks' bit

Also just thanks so much for this pack, it's been one of my favourites, really makes forests just feel a bit more alive <3

I didn't even think of that! thank you ^^