I'm more interested in Java Edition. Feel free to include Bedrock for prosperity, but please be explicit about which edition you're referring to.

I'm trying to place a piston.

But, in my situation, I'm only able to place it from "behind", since my player won't have access to the area needed for the default orientation to work.

Can we place blocks "backwards", facing the opposite direction that they're normally facing when placed? Maybe through some key combination?

Example

Could I place a piston like this, from the position I'm standing in?

Let's take a look at the Minecraft Wiki articles on directional blocks.

Pistons:

Pistons are always placed facing toward the player. When powered...

Droppers:

A dropper can be placed so that its output faces in any direction, including up or down. When placed, the dropper's output faces toward the player.

Dispensers:

A dispenser can be placed so that its output faces in any direction, including up or down. When placed, a dispenser always faces the player placing it.

Unfortunately, what you have requested is not possible in Vanilla Minecraft. You could try creating a complicated command-gadget for this or download a mod that would do the trick, but those are entirely different topics for entirely different posts.

If you're in Java Edition and building in Creative Mode and need to get inside your redstone contraption at the right angle to place your directional blocks, you can press F3+N to switch to Spectator Mode, fly into the redstone contraption, press it again to switch back to Creative, and place the block facing the correct way.

Combining the Tweakeroo and Carpet client and server mods if playing on a server, or just tweakeroo for single-player, you can activate some more placement modes that allow you to do that.