Currently the player has to place a carpenter's block down and then click it again to texture it. The player is effectively building their build twice. While this is easy in small/normal builds, it gets difficult and tedious when building something massive. I suggest that a way to texture more blocks than just the one be implemented. Maybe an item that would texture all connected blocks in a 3x3x3 area?

I kinda agree with this. I just recently started building something with quite a few slopes and having to manually texture each one of hundreds (if not thousands) is pretty daunting. I suppose I understand not wanting to make it OP(coloring everything diamond with one diamond block I.E.) but I would like the option at least. Perhaps make the carpenters blocks paintable with EnderIO's painting machine? That would seem to be the most straightforward way although it relies on the player having EnderIO (why wouldn't you, it's awesome)

I was thinking more like some kind paint gun that has single-slot inventory that the player would have to load the blocks into. More balanced.

That doesn't sound any less tedious. Being able to pre-texture the blocks removes the majority of the hassle. If there was a way to do that while still requiring donor items at a 1:1 ratio that seems like it would be the fastest, most balanced way. It's not going to stop me from using them either way. Just having slopes makes it worth the trouble doing them one at a time.

Not sure how likely this is to work, but one way of doing this could simply be to place the kind of block into a crafting grid with the carpenter block to be painted. That seems like a lot of recipes, though.
A specific block painter machine (simple, cheap to make) might be the way to go. Forex, one would place a stack of the carpenters block blocks in one slot, and a stack of the material used to provide the texture into a second slot.
The problem with using the EnderIO painting machine is that it doesn't use the block provided to make the texture, leading to block duplication ease.

Not to mention that would make the feature only accessible if you have EnderIO

It wouldn't be too difficult to texture nearby blocks if you're carrying more than one of the same item. What I need is another button combination to trigger this feature. An item is a possibility, but would take more time.

I was thinking you could add the ability to pretexture carpenter's blocks by placing them in the crafting grid with an equal amount of the block with which to texture them. Upon breaking they could still yield the carpenter's block and texture block.

In my scarce understanding of modding, this "crafting" would demand a lot of coding, I believe that the mod only uses an small code to "imitate" block properties and texture, so that's why it's so great, and also so low on crafting recipes, what would be a good idea indeed, would be a kind of "wand" or "big hammer" that lets you automaticaly texture a radius of x carpenter blocks with an specified item, similar to wands of thaumcraft when using the equal exchange cap or whatever.