About [[EOL:Website-to/en]]

Yes, kind of, but due to its complexity, it cannot be joined together.

I've actually changed the name of this key to make it slightly clearer what's going on (and because "to" is an AWFUL name, argh) and added this description:

"The value of this key is simply 'to' because it is used in the context of a date range, where the user inputs a start date before this string and an end date after it. ie: '...a user who registered from ____ to ____'"

I do not know about EOL, but in MediaWiki you could do this with placeholders that software replaces with input fields when the message is output. For Example:

Please specify: the event runs from $1 to $2 o'clock.