Is your feature request related to a problem? Please describe.
This request does not necessarily solve a problem, but instead allows for rapid editing of selected blocks based on blocks already in your environment, without typing out the entire block name and associated data. It could potentially assist players in doing block edits even if they do not know the block name or format of the blockstate data, simply by looking at the block they want to use.

Describe the solution you'd like
An example usage of this feature could be as simple as //set #targeted where the current WE selection would be replaced with whatever block is currently displayed in the Minecraft F3 window under Targeted Block or, put more simply, whatever block you are currently looking at. Other examples could be things like //replace #targeted stone or //gmask #targeted

Describe alternatives you've considered
The default raycast length used in the F3 window to query the Targeted Block would be sufficient, but if that raycast length could be extended, it would be even more helpful when using this feature.

In a best case scenario, one would be able to store two different #targeted selections as variables (most likely as part of a CraftScript), then run a command or script like //replace #targeted1 #targeted2 This would allow a builder to do rapid replacements of block selections just by looking at two different blocks and running one command.

Additional context

would note that the targeted stuff in f3 is completely clientside. i think the client might send a location, but it's not exposed everywhere (at least not in bukkit). i'd also note that for things like the pos1 and hand patterns, there's already some potential issues with async gets during tab completion that would be much exacerbated by server-side ray tracing.